config: avoid segfault when parsing command-line config

We already check for an empty key on the left side of an
equals, but we would segfault if there was no content at
all.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1300-repo-config.sh

@@ -918,4 +918,8 @@ test_expect_success 'git -c complains about empty key' '
 	test_must_fail git -c "=foo" rev-parse
 '
 
+test_expect_success 'git -c complains about empty key and value' '
+	test_must_fail git -c "" rev-parse
+'
+
 test_done