refine a dog_eat impossible()

Have the impossible message indicate whether the pointed-to
edog struct itself is still intact (not overwritten somewhere,
such as misuse of a stale or bad pointer) versus the apport field
itself being assigned an out-of-whack value some place.

src/dogmove.c

@@ -316,9 +316,13 @@ dog_eat(struct monst *mtmp,
             edog->apport += (int) (200L / ((long) edog->dropdist + svm.moves
                                            - edog->droptime));
             if (edog->apport <= 0) {
-                impossible("dog_eat: pet apport <= 0 (%d, %d, %ld, %ld)",
+                impossible("dog_eat: pet apport <= 0 (%d, %d, %ld, %ld, %ud, %ud)",
                             edog->apport, edog->dropdist, edog->droptime,
-                            svm.moves);
+                            svm.moves,
+                           /* check whether edog struct got clobbered;
+                              these two values should always match if
+                              edog content is still intact */
+                           mtmp->m_id, edog->parentmid);
                 edog->apport = 1;
             }
         }