Revert "lzx-compress.c: Simplify calculation of position footer"

Revert "lzx-compress.c: Simplify calculation of position footer"

This reverts commit 3adc1ac1ebe221427857d8f6fd06cfb823b4bea6.

If the position footer is unconditionally calculated as the match offset
minus the position base value, the (ultimately unused) position footer
for repeat matches can overflow the number of bits in which it is stored
in the intermediate representation used by this implementation.  For now,
use the old version, which would set the position footers of repeat
matches to 0.