Add assert for potential access beyond array bounds in m1np

Add assert for potential access beyond array bounds in m1np

The mpexp code has an access into m1np:

  for (i=n-1; i>0; i--,n--) { if (m1np[i][p]+m2>0) break; }

which could break for p >= 18 or i >= 7.  Fortunately this code is
never called due to the way the exp function is implemented since
values having exponent less than -55 return 1.0.  Make sure that if it
gets called in future, it is trapped.