Avoided merging runlists with no runs

Runlists with no runs are tolerated though not expected. However merging
such runlists is problematic as there is no significant vcn to examine.
So avoid merging them, and just return the other runlist.

libntfs-3g/runlist.c

@@ -994,13 +994,18 @@ static runlist_element *ntfs_mapping_pairs_decompress_i(const ntfs_volume *vol,
 	rl[rlpos].vcn = vcn;
 	rl[rlpos].length = (s64)0;
 	/* If no existing runlist was specified, we are done. */
-	if (!old_rl) {
+	if (!old_rl || !old_rl[0].length) {
 		ntfs_log_debug("Mapping pairs array successfully decompressed:\n");
 		ntfs_debug_runlist_dump(rl);
+		if (old_rl)
+			free(old_rl);
 		return rl;
 	}
 	/* Now combine the new and old runlists checking for overlaps. */
-	old_rl = ntfs_runlists_merge(old_rl, rl);
+	if (rl[0].length)
+		old_rl = ntfs_runlists_merge(old_rl, rl);
+	else
+		free(rl);
 	if (old_rl)
 		return old_rl;
 	err = errno;