builtin/checkout--worker: zero-initialise struct to avoid MSAN complaints

report_result() sends a struct to the parent process, but that struct
would contain uninitialised padding bytes. Running this code under MSAN
rightly triggers a warning - but we don't particularly care about this
warning because we control the receiving code, and we therefore know
that those padding bytes won't be read on the receiving end.

We could simply suppress this warning under MSAN with the approporiate
ifdef'd attributes, but a less intrusive solution is to 0-initialise the
struct, which guarantees that the padding will also be initialised.

Interestingly, in the error-case branch, we only try to copy the first
two members of pc_item_result, by copying only PC_ITEM_RESULT_BASE_SIZE
bytes. However PC_ITEM_RESULT_BASE_SIZE is defined as
'offsetof(the_last_member)', which means that we're copying padding bytes
after the end of the second last member. We could avoid doing this by
redefining PC_ITEM_RESULT_BASE_SIZE as
'offsetof(second_last_member) + sizeof(second_last_member)', but there's
no huge benefit to doing so (and this patch silences the MSAN warning in
this scenario either way).

MSAN output from t2080 (partially interleaved due to the
parallel work :) ):

Uninitialized bytes in __interceptor_write at offset 12 inside [0x7fff37d83408, 160)
==23279==WARNING: MemorySanitizer: use-of-uninitialized-value
Uninitialized bytes in __interceptor_write at offset 12 inside [0x7ffdb8a07ec8, 160)
==23280==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0xd5ac28 in xwrite /home/ahunt/git/git/wrapper.c:256:8
    #1 0xd5b327 in write_in_full /home/ahunt/git/git/wrapper.c:311:21
    #2 0xb0a8c4 in do_packet_write /home/ahunt/git/git/pkt-line.c:221:6
    #3 0xb0a5fd in packet_write /home/ahunt/git/git/pkt-line.c:242:6
    #4 0x4f7441 in report_result /home/ahunt/git/git/builtin/checkout--worker.c:69:2
    #5 0x4f6be6 in worker_loop /home/ahunt/git/git/builtin/checkout--worker.c💯3
    #6 0x4f68d3 in cmd_checkout__worker /home/ahunt/git/git/builtin/checkout--worker.c:143:2
    #7 0x4a1e76 in run_builtin /home/ahunt/git/git/git.c:461:11
    #8 0x49e1e7 in handle_builtin /home/ahunt/git/git/git.c:714:3
    #9 0x4a0c08 in run_argv /home/ahunt/git/git/git.c:781:4
    #10 0x49d5a8 in cmd_main /home/ahunt/git/git/git.c:912:19
    #11 0x7974da in main /home/ahunt/git/git/common-main.c:52:11
    #12 0x7f8778114349 in __libc_start_main (/lib64/libc.so.6+0x24349)
    #13 0x421bd9 in _start /home/abuild/rpmbuild/BUILD/glibc-2.26/csu/../sysdeps/x86_64/start.S:120

  Uninitialized value was created by an allocation of 'res' in the stack frame of function 'report_result'
    #0 0x4f72c0 in report_result /home/ahunt/git/git/builtin/checkout--worker.c:55

SUMMARY: MemorySanitizer: use-of-uninitialized-value /home/ahunt/git/git/wrapper.c:256:8 in xwrite
Exiting
    #0 0xd5ac28 in xwrite /home/ahunt/git/git/wrapper.c:256:8
    #1 0xd5b327 in write_in_full /home/ahunt/git/git/wrapper.c:311:21
    #2 0xb0a8c4 in do_packet_write /home/ahunt/git/git/pkt-line.c:221:6
    #3 0xb0a5fd in packet_write /home/ahunt/git/git/pkt-line.c:242:6
    #4 0x4f7441 in report_result /home/ahunt/git/git/builtin/checkout--worker.c:69:2
    #5 0x4f6be6 in worker_loop /home/ahunt/git/git/builtin/checkout--worker.c💯3
    #6 0x4f68d3 in cmd_checkout__worker /home/ahunt/git/git/builtin/checkout--worker.c:143:2
    #7 0x4a1e76 in run_builtin /home/ahunt/git/git/git.c:461:11
    #8 0x49e1e7 in handle_builtin /home/ahunt/git/git/git.c:714:3
    #9 0x4a0c08 in run_argv /home/ahunt/git/git/git.c:781:4
    #10 0x49d5a8 in cmd_main /home/ahunt/git/git/git.c:912:19
    #11 0x7974da in main /home/ahunt/git/git/common-main.c:52:11
    #12 0x7f2749a0e349 in __libc_start_main (/lib64/libc.so.6+0x24349)
    #13 0x421bd9 in _start /home/abuild/rpmbuild/BUILD/glibc-2.26/csu/../sysdeps/x86_64/start.S:120

  Uninitialized value was created by an allocation of 'res' in the stack frame of function 'report_result'
    #0 0x4f72c0 in report_result /home/ahunt/git/git/builtin/checkout--worker.c:55

SUMMARY: MemorySanitizer: use-of-uninitialized-value /home/ahunt/git/git/wrapper.c:256:8 in xwrite

Signed-off-by: Andrzej Hunt <andrzej@ahunt.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout--worker.c

@@ -53,7 +53,7 @@ static void packet_to_pc_item(const char *buffer, int len,
 
 static void report_result(struct parallel_checkout_item *pc_item)
 {
-	struct pc_item_result res;
+	struct pc_item_result res = { 0 };
 	size_t size;
 
 	res.id = pc_item->id;