crt: Check the return value from MultiByteToWideChar in btowc

If MultiByteToWideChar failed, it could still have overwritten
the output variable.

Signed-off-by: Martin Storsjö <martin@martin.st>

mingw-w64-crt/misc/btowc.c

@@ -19,8 +19,10 @@ wint_t btowc (int c)
     {
       unsigned char ch = c;
       wchar_t wc = WEOF;
-      MultiByteToWideChar (___lc_codepage_func(), MB_ERR_INVALID_CHARS,
-			   (char*)&ch, 1, &wc, 1);
+      if (!MultiByteToWideChar (___lc_codepage_func(), MB_ERR_INVALID_CHARS,
+                                (char*)&ch, 1, &wc, 1))
+        return WEOF;
+
       return wc;
     }
 }