[tsan] Change nullptr to NULL in realloc-zero.cc test. Some environments don't have nullptr.

llvm-svn: 286166

compiler-rt/test/tsan/Darwin/realloc-zero.cc

@@ -9,7 +9,7 @@
 #include <sys/mman.h>
 
 int main() {
-  void *p = realloc(nullptr, 0);
+  void *p = realloc(NULL, 0);
   if (!p) {
     abort();
   }