pacify static analyzer - dog.c

pacify static analyzer - dog.c

This is similar to the earlier potential fix that I didn't like,
but I think think one is better.

The analyzer claimed that 'fptr' might be Null inside the switch
case for
|struct permonst *fptr = NULL;
|if (obj->otyp == CORPSE || ...) fptr = &mons[obj->corpsenm];
|switch (obj->otyp) { case CORPSE: ... /* dereference 'fptr' */ }
even though it will always have a non-Null value for otyp==CORPSE.

Make the assignment of 'fptr' unconditional.  mons[NUMMONS] is
valid and won't match any actual monster.  In this case it will
only be used when initializing fptr, never when fptr gets used.