checkpatch: check Signed-off-by in --mailback mode

Pull the test before the anticipated exits from the process sub.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

scripts/checkpatch.pl

@@ -2864,6 +2864,10 @@ sub process {
 		}
 	}
 
+	if ($is_patch && $chk_signoff && $signoff == 0) {
+		ERROR("Missing Signed-off-by: line(s)\n");
+	}
+
 	# If we have no input at all, then there is nothing to report on
 	# so just keep quiet.
 	if ($#rawlines == -1) {
@@ -2885,9 +2889,6 @@ sub process {
 	if (!$is_patch) {
 		ERROR("Does not appear to be a unified-diff format patch\n");
 	}
-	if ($is_patch && $chk_signoff && $signoff == 0) {
-		ERROR("Missing Signed-off-by: line(s)\n");
-	}
 
 	print report_dump();
 	if ($summary && !($clean == 1 && $quiet == 1)) {