Fix xattr error test.

toys/posix/tar.c

diff --git a/toys/posix/tar.c b/toys/posix/tar.c
index abc884b..06482c1 100644
--- a/toys/posix/tar.c
+++ b/toys/posix/tar.c
@@ -406,7 +406,7 @@
           strcpy(buf+start+sz-1, "\n");
           write_prefix_block(buf, start+sz, 'x');
         } else if (errno==ENODATA || errno==ENOTSUP) len = 0;
-        if (len) perror_msg("getfilecon %s", name);
+        if (len<0) perror_msg("getfilecon %s", name);
 
         free(buf);
         break;