git-fetch: Don't trigger a bus error when given the refspec "tag"

When git-fetch encounters the refspec "tag" it assumes that the next
argument will be a tag name. If there is no next argument, it should
die gracefully instead of erroring.

Signed-off-by: Kevin Ballard <kevin@sb.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-fetch.c

@@ -637,6 +637,8 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 			if (!strcmp(argv[i], "tag")) {
 				char *ref;
 				i++;
+				if (i >= argc)
+					die("You need to specify a tag name.");
 				ref = xmalloc(strlen(argv[i]) * 2 + 22);
 				strcpy(ref, "refs/tags/");
 				strcat(ref, argv[i]);