x86: Fix off-by-one error in do_elf_reloc_fixups()

The use of post-increment with a do-while loop results in
the loop going one step too far when handling relocation fixups.

In about 1/100 cases this would cause it to hang.

Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>