Fix: Cleaver's should_cleave could check off map

It is possible for the player to hit a monster with Cleaver such that
spaces in the arc are not on the map. The should_cleave function didn't
take this into account, so it was possible to segfault by looking for a
monster there. Add an isok() check to prevent that.

Thanks to paxed for pointing out the bug and qt for finding a reliable
way to make it segfault.

src/uhitm.c

@@ -620,7 +620,12 @@ should_cleave(void)
      * trigger this prompt */
     for (i = dir + 1; i <= dir + 7; i += 6) {
         int realdir = i % 8;
-        struct monst *mtmp = m_at(u.ux + xdir[realdir], u.uy + ydir[realdir]);
+        int x = u.ux + xdir[realdir];
+        int y = u.uy + ydir[realdir];
+        struct monst *mtmp;
+        if (!isok(x, y))
+            continue;
+        mtmp = m_at(x, y);
         if (mtmp && canspotmon(mtmp) && mtmp->mpeaceful) {
             bystanders = TRUE;
         }