fix issue #1434 - engulfed gas spore explosion

Issue reported by Umbire:  a gas spore that got swallowed and killed
didn't die but exploded anyway, with the explosion affecting the map
instead of being contained in the swallower.

There was code to handle that but it wasn't being executed.  This fix
feels unclean but seems to work.

I couldn't reproduce the survival of the gas spore but since that
isn't wanted I won't worry about it.

Fixes #1434

src/mhitm.c

@@ -906,7 +906,9 @@ gulpmm(
     newsym(ax, ay); /* erase old position */
     newsym(dx, dy); /* update new position */
 
+    gm.mswallower = magr; /* corpse_chance() wants this */
     status = mdamagem(magr, mdef, mattk, (struct obj *) 0, 0);
+    gm.mswallower = (struct monst *) 0; /* reset */
 
     if ((status & (M_ATTK_AGR_DIED | M_ATTK_DEF_DIED))
         == (M_ATTK_AGR_DIED | M_ATTK_DEF_DIED)) {