ui-patch: make sure to send http headers

Requesting a text/plain patch with bad commit id made cgit send text
without proper http headers. This results in "500 Internal Server Error"
with "Premature end of script headers" in server logs.
So print http headers before error message and return.

Signed-off-by: Christian Hesse <mail@eworm.de>
Reviewed-by: John Keeping <john@keeping.me.uk>

ui-patch.c

@@ -25,21 +25,25 @@ void cgit_print_patch(const char *new_rev, const char *old_rev,
 		new_rev = ctx.qry.head;
 
 	if (get_sha1(new_rev, new_rev_sha1)) {
+		cgit_print_http_headers();
 		cgit_print_error("Bad object id: %s", new_rev);
 		return;
 	}
 	commit = lookup_commit_reference(new_rev_sha1);
 	if (!commit) {
+		cgit_print_http_headers();
 		cgit_print_error("Bad commit reference: %s", new_rev);
 		return;
 	}
 
 	if (old_rev) {
 		if (get_sha1(old_rev, old_rev_sha1)) {
+			cgit_print_http_headers();
 			cgit_print_error("Bad object id: %s", old_rev);
 			return;
 		}
 		if (!lookup_commit_reference(old_rev_sha1)) {
+			cgit_print_http_headers();
 			cgit_print_error("Bad commit reference: %s", old_rev);
 			return;
 		}