Fix the check-model step under SBCL.

Fix various problem with the check-model step when PVS is compiled against
SBCL:
 - Wrap the mu___append_cont so that 32bit numbers are converted to pointers.
Otherwise SBCL throws type errors.  To avoid compatibility issues, the alien
interface isn't changed.
 - Fix comparisons involving c pointers.
 - Properly check for null pointers, using an implementation proper way.  SBCL
requires a proper null check instead of just comparing the pointer to zero.

Note I only tried running this under SBCL.  Except for the check-null function, I don't think anything else should be broken.