another static analyzer bit for mkobj.c

src/mkobj.c(419): warning: '((obj2))->oextra->omonst' could be '0'
                : this does not adhere to the specification for the
                  function 'memcpy'.
src/mkobj.c(421): warning: Dereferencing NULL pointer
                  '((obj2))->oextra->omonst'.
                  See line 419 for an earlier location where this can occur

The analyzer was not aware that newoextra() sets up an oextra block:
    if (!obj2->oextra)
        obj2->oextra = newoextra();

The analyzer was also not aware that newomonst() was setting up a valid
OMONST pointer.
    if (!OMONST(obj2))
        newomonst(obj2);

Add an assert(has_omonst(obj2)) before copying the content from
OMONST(obj1) into OMONST(obj2).

src/mkobj.c

@@ -416,6 +416,7 @@ copy_oextra(struct obj *obj2, struct obj *obj1)
     if (has_omonst(obj1)) {
         if (!OMONST(obj2))
             newomonst(obj2);
+        assert(has_omonst(obj2));
         (void) memcpy((genericptr_t) OMONST(obj2),
                       (genericptr_t) OMONST(obj1), sizeof (struct monst));
         OMONST(obj2)->mextra = (struct mextra *) 0;