[dfsan] Set sigemptyset's return label to be 0

This was not set from when the wrapper was introduced.

Reviewed By: gbalats

Differential Revision: https://reviews.llvm.org/D99678

Author: stephan-yichao-zhao

compiler-rt/lib/dfsan/dfsan_custom.cpp

@@ -1573,6 +1573,7 @@ int __dfsw_sigemptyset(sigset_t *set, dfsan_label set_label,
                        dfsan_label *ret_label) {
   int ret = sigemptyset(set);
   dfsan_set_label(0, set, sizeof(sigset_t));
+  *ret_label = 0;
   return ret;
 }