gzip: fix error handling in unxz

The result of fstat() was not checked.  Furthermore, there was a
redundant check of st.st_size.  Fix both.

Reported by:    Coverity
MFC after:	1 week
Sponsored by:	Dell EMC Isilon

usr.bin/gzip/unxz.c

@@ -260,8 +260,7 @@ parse_indexes(xz_file_info *xfi, int src_fd)
 {
 	struct stat st;
 
-	fstat(src_fd, &st);
-	if (st.st_size <= 0) {
+	if (fstat(src_fd, &st) != 0) {
 		return true;
 	}