Fix zapping cancellation at magical trap

The exploding magical trap did not cause problems before just by
being a magical explosion by accident; adding an object caused
the cancellation wand ID to change, and the explosion now had
an impossible explosion type.

Hardcode trap explosions to generic magical blasts.

src/explode.c

@@ -259,6 +259,8 @@ explode(
     } else if (olet == SCROLL_CLASS) {
         /* ditto */
         exploding_wand_typ = SCR_FIRE;
+    } else if (olet == TRAP_EXPLODE) {
+        type = 0; /* hardcoded to generic magic explosion */
     }
     /* muse_unslime: SCR_FIRE */
     if (expltype < 0) {