fix(mutex): safely call snprintf

In the PostSynchEvent() function, the pos integer uses an implementation
of snprintf that is fundamentally unsafe: since the return value of
snprintf is the number of characters that would have been written to the
buffer, if an operation reaches the end of the buffer with more than one
character discarded, the return value will be greater than the buffer
size, requiring a check of the buffer's current size.

Signed-off-by: Elijah Conners <business@elijahpepe.com>

absl/synchronization/mutex.cc

@@ -430,7 +430,11 @@ static void PostSynchEvent(void *obj, int ev) {
     char buffer[ABSL_ARRAYSIZE(pcs) * 24];
     int pos = snprintf(buffer, sizeof (buffer), " @");
     for (int i = 0; i != n; i++) {
-      pos += snprintf(&buffer[pos], sizeof (buffer) - pos, " %p", pcs[i]);
+      int b += snprintf(&buffer[pos], sizeof (buffer) - pos, " %p", pcs[i]);
+      if (b < 0 || b >= sizeof (buffer) - pos) {
+        break;
+      }
+      pos += b;
     }
     ABSL_RAW_LOG(INFO, "%s%p %s %s", event_properties[ev].msg, obj,
                  (e == nullptr ? "" : e->name), buffer);