[sanitizer] Avoid memset call in tsan

[rupprecht]

Summary

Assigning an array of bools to {false, false, false} can, in certain build configurations, lead to a memset call. Use internal_memset to avoid this.

[rupprecht]

Evgenii -- I'd like to commit now to fix our internal breakage (this looks trivial enough to me), but I'm happy to do post-commit changes.

[eugenis]

LGTM

lib/sanitizer_common/sanitizer_posix.cc

const char *SignalContext::Describe() const {
   }
   return "UNKNOWN SIGNAL";
 }
 
 fd_t ReserveStandardFds(fd_t fd) {
   CHECK_GE(fd, 0);
   if (fd > 2)
     return fd;
-  bool used[3] = {false, false, false};
+  bool used[3];
+  internal_memset(used, 0, sizeof(used));
   while (fd <= 2) {
     used[fd] = true;
     fd = internal_dup(fd);
   }
   for (int i = 0; i <= 2; ++i)
     if (used[i])
       internal_close(i);
   return fd;
 }
 
 } // namespace __sanitizer
 
 #endif // SANITIZER_POSIX