Fix segfault when calling git_config_get_* functions when a config fails to load

Reinitialize the result code of get_entry() to GIT_ENOTFOUND

src/config.c

@@ -651,6 +651,7 @@ static int get_entry(
 		key = normalized;
 	}
 
+	res = GIT_ENOTFOUND;
 	git_vector_foreach(&cfg->files, i, internal) {
 		if (!internal || !internal->file)
 			continue;