add a pair of shorthand macros to validate an index into an array

Two variations:

IndexOk(idx, array)       validate that idx is a valid index into the array

IndexOkT(idx, array)      validate that idx is a valid index into the
                          array, excluding the final Terminator element

src/monmove.c

@@ -1817,7 +1817,7 @@ m_move(register struct monst *mtmp, int after)
          * mfndpos) has no effect for normal attacks, though it lets a
          * confused monster attack you by accident.
          */
-        assert(chi >= 0 && chi < SIZE(info));
+        assert(IndexOk(chi, info));
         if (info[chi] & ALLOW_U) {
             nix = mtmp->mux;
             niy = mtmp->muy;