Fix div by zero or overflow detection on Unix

Fix div by zero or overflow detection on Unix

This change fixes a bug in the code that detects whether a division error
is a division by zero or arithmetic overflow. The code was incorrectly
handling instructions with the SIB byte, since the rm was or-ed with the
rex_r bit even for the case when there was a SIB byte, which in case
where the rex_r was set made it miss the SIB byte.