Zero-initialize stat structures before fstat calls.

This is upstreaming a fix from Google.

MSan reports use-of-uninitialized-value in google_breakpad::MemoryMappedFile::Map 
because it cannot track memory initialized via syscalls like sys_fstat.

Reintroducing zero-initialization using __builtin_memset to avoid 
introducing a dependency on the standard library in this low-level context.

Bug: 504133260
Change-Id: Ic9fd0a0173e74e0308f2f4c8f3e96fc039b846a2
Reviewed-on: https://chromium-review.googlesource.com/c/breakpad/breakpad/+/7780286
Reviewed-by: Joshua Peraza <jperaza@chromium.org>

Author: ivanpenkov

src/common/linux/memory_mapped_file.cc

@@ -75,9 +75,11 @@ bool MemoryMappedFile::Map(const char* path, size_t offset) {
    (defined(__riscv) && __riscv_xlen == 64)
 
   struct kernel_stat st;
+  __builtin_memset(&st, 0, sizeof(st));
   if (sys_fstat(fd, &st) == -1 || st.st_size < 0) {
 #else
   struct kernel_stat64 st;
+  __builtin_memset(&st, 0, sizeof(st));
   if (sys_fstat64(fd, &st) == -1 || st.st_size < 0) {
 #endif
     sys_close(fd);