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/mhitu.c

@@ -1271,7 +1271,7 @@ gulp_blnd_check(void)
     return FALSE;
 }
 
-/* monster swallows you, or damage if u.uswallow */
+/* monster swallows you, or damage if already swallowed (u.uswallow != 0) */
 staticfn int
 gulpmu(struct monst *mtmp, struct attack *mattk)
 {
@@ -1537,7 +1537,9 @@ gulpmu(struct monst *mtmp, struct attack *mattk)
     if (physical_damage)
         tmp = Maybe_Half_Phys(tmp);
 
+    gm.mswallower = mtmp; /* match gulpmm() */
     mdamageu(mtmp, tmp);
+    gm.mswallower = 0;
     if (tmp)
         stop_occupation();