Fix hero hiding under a statue shattered by land mine

Trigger a land mine while being polymorphed into a monster that
automatically hides (eg. scorpion). Have the statue on the land mine
shatter and all items on that square scatter away. Avoid falling
into the pit left by the land mine.

src/explode.c

@@ -771,6 +771,9 @@ struct obj *obj; /* only scatter this obj        */
         newsym(x, y);
     }
     newsym(sx, sy);
+    if (sx == u.ux && sy == u.uy && u.uundetected
+        && hides_under(youmonst.data))
+        (void) hideunder(&youmonst);
     return total;
 }