Fix the fix for the native symbol resolver on Windows.

runtime/vm/native_symbol_win.cc

Index: runtime/vm/native_symbol_win.cc
diff --git a/runtime/vm/native_symbol_win.cc b/runtime/vm/native_symbol_win.cc
index 78cbe8f370e477f8712a2082c6e02a2791101753..80cf3e0c290d8f6145c0c2de01b7a23e477683df 100644
--- a/runtime/vm/native_symbol_win.cc
+++ b/runtime/vm/native_symbol_win.cc
@@ -67,7 +67,9 @@ char* NativeSymbolResolver::LookupSymbolName(uintptr_t pc, uintptr_t* start) {
   if (r == FALSE) {
     return NULL;
   }
-  *start = pc - displacement;
+  if (start != NULL) {
+    *start = pc - displacement;
+  }
   return strdup(pSymbol->Name);
 }