diff: make sure --output=/bad/path is caught

The return value from fopen wasn't being checked.
Signed-off-by: NLarry D'Anna <larry@elder-gods.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

diff.c

@@ -2799,6 +2799,8 @@ int diff_opt_parse(struct diff_options *options, const char **av, int ac)
 		;
 	else if (!prefixcmp(arg, "--output=")) {
 		options->file = fopen(arg + strlen("--output="), "w");
+		if (!options->file)
+			die_errno("Could not open '%s'", arg + strlen("--output="));
 		options->close_file = 1;
 	} else
 		return 0;