Impossible timer after vault guard entered

Fuzzer encountered impossible "timer sanity: melt timer on non-ice",
when vault guard entered the vault, changing the vault wall unconditionally
into room floor; in this case the vault wall was ice with a melting timer
attached.

Delete any melting ice timers on that location when turning it into floor.

src/vault.c

@@ -608,6 +608,7 @@ invault(void)
         EGD(guard)->fakecorr[0].ftyp = typ;
         EGD(guard)->fakecorr[0].flags = levl[x][y].flags;
         /* guard's entry point where confrontation with hero takes place */
+        spot_stop_timers(x, y, MELT_ICE_AWAY);
         levl[x][y].typ = DOOR;
         levl[x][y].doormask = D_NODOOR;
         unblock_point(x, y); /* empty doorway doesn't block light */