merge-recursive: add some defensive coding to was_dirty()

It took this developer quite a good while to understand why the current
code cannot get a `NULL` returned by `index_file_exists()`. To
un-confuse readers (and future-proof the code), let's just be safe and
check before we dereference the returned pointer.

merge-recursive.c

@@ -870,7 +870,7 @@ static int was_dirty(struct merge_options *opt, const char *path)
 
 	ce = index_file_exists(opt->priv->unpack_opts.src_index,
 			       path, strlen(path), ignore_case);
-	return verify_uptodate(ce, &opt->priv->unpack_opts) != 0;
+	return !ce || verify_uptodate(ce, &opt->priv->unpack_opts) != 0;
 }
 
 static int make_room_for_path(struct merge_options *opt, const char *path)