preproc: don't loop on a variable that doesn't advance
preproc: don't loop on a variable that doesn't advance

When pasting and stripping %+ and whitespace tokens, we either need to
set *nextp in the loop, or treat next as a separate variable and
update *nextp after the loop finishes. This implements the second
option.

This fixes travis test "amx".

Signed-off-by: H. Peter Anvin (Intel) <8a453bad9912ffe59bc0f0b8abe03df9be19379e@zytor.com>

techkey/nasm,techkey/nasm,techkey/nasm,techkey/nasm,techkey/nasm