tar2qfile: Break early if directory headers sent already

There is no reason to continue a loop if subsequent iterations will not
have useful effects.

(cherry picked from commit 1a2cd93)

qubes-rpc/tar2qfile.c

@@ -750,6 +750,7 @@ ustar_rd (int fd, struct file_header * untrusted_hdr, char *buf, struct stat * s
 				fprintf(stderr,"Directory headers already sent\n");
 #endif
 				dir_found=1;
+				break;
 			}
 		}
 		if (dir_found == 0) {