wrapper.c: add and use fopen_or_warn()

When fopen() returns NULL, it could be because the given path does not
exist, but it could also be some other errors and the caller has to
check. Add a wrapper so we don't have to repeat the same error check
everywhere.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/remote.c b/remote.c
index 801137c..1f2453d 100644
--- a/remote.c
+++ b/remote.c
@@ -251,7 +251,7 @@ static const char *skip_spaces(const char *s)
 static void read_remotes_file(struct remote *remote)
 {
 	struct strbuf buf = STRBUF_INIT;
-	FILE *f = fopen(git_path("remotes/%s", remote->name), "r");
+	FILE *f = fopen_or_warn(git_path("remotes/%s", remote->name), "r");
 
 	if (!f)
 		return;
@@ -277,7 +277,7 @@ static void read_branches_file(struct remote *remote)
 {
 	char *frag;
 	struct strbuf buf = STRBUF_INIT;
-	FILE *f = fopen(git_path("branches/%s", remote->name), "r");
+	FILE *f = fopen_or_warn(git_path("branches/%s", remote->name), "r");
 
 	if (!f)
 		return;