parse_fline: fix implicit cast sign

(cherry picked from commit b0eb928)

parser/parse_fline.c

@@ -1275,7 +1275,7 @@ char* parse_first_line(char* buffer, unsigned int len, struct msg_start * fl)
 	if (prn) {
 		for (t=0; t<offset; t++)
 			if (*(buffer+t)) *(prn+t)=*(buffer+t);
-			else *(prn+t)=248;
+			else *(prn+t)=248U;
 		LM_ERR("parsed so far: %.*s\n", offset, ZSW(prn) );
 		pkg_free( prn );
 	};