Fix: trapped without a trap (fmon). This is something strange I picke…

…d up in the fuzzer. A green slime was able to engulf a monster while being trapped in a pit. After finishing the engulf attack, the slime was placed outside of the trap but still was marked as trapped.

src/steed.c

@@ -998,6 +998,12 @@ int x, y;
                    mon->mstate, buf);
         return;
     }
+    /* If monster was trapped, it's possible that while engulfing it
+     * may change position. Ex: A green slime engulfing a monster can end
+     * up in a different position than it started after finishing the engulf. */
+    if (mon->mtrapped && mon->mx != x && mon->my != y)
+        mon->mtrapped = 0;
+
     mon->mx = x, mon->my = y;
     level.monsters[x][y] = mon;
 }