math/mprand.[ch], rand/grand.c: Check range of arguments.

math/mprand.[ch], rand/grand.c: Check range of arguments.

  * mprand: It doesn't make sense to ask for a zero-bit integer whose
    low bit is set; or, indeed, a four-bit integer whose fourth bit is
    set.  So check the mask against the bit length.

  * mprand: On the other hand, it /does/ make sense to ask for a
    zero-bit integer, and the answer is simply zero.  But the code used
    to segfault.  Fix this.

  * mprand_range, grand_defaultrange: It doesn't make sense to ask for
    an integer in [0, 0), because there aren't any.  Check before
    trying.