Address infinite loop when processing $GENERATE

Address infinite loop when processing $GENERATE

In nibble mode if the value to be converted was negative the parser
would loop forever.  Process the value as an unsigned int instead
of as an int to prevent sign extension when shifting.

This was found by Eric Sesterhenn from X41.

(cherry picked from commit 371824f0789d6e491216f266bf62955a73b49858)