Fix squeaky board creation segfault

If a trap is created on top of another trap, maketrap reuses
the trap struct in place, instead of deleting and recreating it.

If a squeaky trap was created on top of another trap, maketrap
first set the trap type to squeaky board, and then tried to
look through all squeaky boards on the level, to determine
what note the new trap should play. Unfortunately, the union
with the trap note most likely contained a rolling boulder
coordinate or something else outside the 12 note range, so
then the tavail-array lookup would cause a segfault.

src/trap.c

@@ -339,7 +339,7 @@ register int x, y, typ;
         for (k = 0; k < 12; ++k)
             tavail[k] = 0;
         for (t = ftrap; t; t = t->ntrap)
-            if (t->ttyp == SQKY_BOARD)
+            if (t->ttyp == SQKY_BOARD && t != ttmp)
                 tavail[t->tnote] = 1;
 
         /* Now populate tpick with the available indexes */