libarchive: fix xmalloc_open_zipped_read_close() on NOMMU

The somewhat new "unpack in memory" code was broken
for xmalloc_open_zipped_read_close() on NOMMU: we seek back
over signature, but then expect it to be already consumed.
Stop seeking back in this case.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/archival/libarchive/decompress_unxz.c b/archival/libarchive/decompress_unxz.c
index 1f408ab..cd32cc7 100644
--- a/archival/libarchive/decompress_unxz.c
+++ b/archival/libarchive/decompress_unxz.c
@@ -55,7 +55,7 @@
 	iobuf.out = membuf + BUFSIZ;
 	iobuf.out_size = BUFSIZ;
 
-	if (!xstate || xstate->check_signature == 0) {
+	if (!xstate || xstate->signature_skipped) {
 		/* Preload XZ file signature */
 		strcpy((char*)membuf, HEADER_MAGIC);
 		iobuf.in_size = HEADER_MAGIC_SIZE;