Checking for a return value with FormatMessage; if the call fails, th…

…ere's no guarantee that the buffer will be non-null.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195019 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/Windows/Windows.h

@@ -39,11 +39,16 @@ inline bool MakeErrMsg(std::string* ErrMsg, const std::string& prefix) {
   if (!ErrMsg)
     return true;
   char *buffer = NULL;
-  FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER|FORMAT_MESSAGE_FROM_SYSTEM,
-      NULL, GetLastError(), 0, (LPSTR)&buffer, 1, NULL);
-  *ErrMsg = prefix + buffer;
+  DWORD R = FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER |
+                          FORMAT_MESSAGE_FROM_SYSTEM,
+                          NULL, GetLastError(), 0, (LPSTR)&buffer, 1, NULL);
+  if (R)
+    *ErrMsg = prefix + buffer;
+  else
+    *ErrMsg = prefix + "Unknown error";
+
   LocalFree(buffer);
-  return true;
+  return R != 0;
 }
 
 template <typename HandleTraits>