checkpatch.pl: Force raw_line to return a defined value

Fixes the warning:
Use of uninitialized value in concatenation (.) or string at
util/lint/checkpatch.pl line 4739

Change-Id: Idc3c631735a595517d77cb8b8ec67e1ac00b6685
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/16357
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

util/lint/checkpatch.pl

@@ -1495,7 +1495,13 @@ sub raw_line {
 		$cnt--;
 	}
 
-	return $line;
+	# coreboot: This probably shouldn't happen, but it does.
+	# Return a defined value so we don't get an error.
+	if (defined $line) {
+		return $line;
+	} else {
+		return "";
+	}
 }
 
 sub cat_vet {