Fix integer input with leading zeros in power-of-two base.

Fix integer input with leading zeros in power-of-two base.

Reading leading '0' characters in integers of base 2, 4, 8, 16, or 32
could result in a NUDS with leading zero digits. This is against the
rules. The result was a misbehaving cl_I down the road.

Thanks to Morgan Deters <mdeters@cs.nyu.edu> of the CVC4 team.