libarchive: treat one "FIXME: avoid seek", take 2

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/archival/libarchive/open_transformer.c b/archival/libarchive/open_transformer.c
index 97bcc32..775bb58 100644
--- a/archival/libarchive/open_transformer.c
+++ b/archival/libarchive/open_transformer.c
@@ -352,13 +352,13 @@
 		 *   read(4, "LF\2\1\1\0\0\0\0"...
 		 * ...and we avoided seeking on the fd! :)
 		 */
-		xstate->signature_skipped = 0;
 		image = xmalloc_read_with_initial_buf(
 			xstate->src_fd,
 			maxsz_p,
 			xmemdup(&xstate->magic, xstate->signature_skipped),
 			xstate->signature_skipped
 		);
+		xstate->signature_skipped = 0;
 	}
 
 	if (!image)