Fix detection of LZMA stream offset in GRUB2

1. Apparently current GCC adds multiple LEA instructions for .p2align,
where exact instruction format depends at least on number pf pad bytes. So
try to be more liberal and just check whether next opcode is LEA (8d). It
catches openSUSE 12.2, openSUSE 13.1 and GRUB 2.02.

2. Pad bytes are not always zeroes, so remove this check.