Fix: archfiend bribe() could get called when hero had no gold

Fuzzer identified an edge case that fell through the cracks and hit the
impossible in money2mon: if an archfiend demanded at least one item from
you and you turned it over, then had no cash, it would attempt to get a
bribe and transfer 0 zorkmids to the monster.

It doesn't appear that bribe is intended to be called with 0 gold since
vanilla checks for this everywhere it's called. So I didn't add any
guard case into bribe() itself; the fix is just to insert an additional
pline noting you have no money instead of calling bribe().

Author: copperwater

src/minion.c

@@ -523,7 +523,11 @@ demon_talk(struct monst *mtmp)
         else if (canseemon(mtmp))
             pline("%s seems to be demanding your money.", Amonnam(mtmp));
 
-        offer = bribe(mtmp);
+        if (cash < 1)
+            pline("But you have no money.");
+        else
+            offer = bribe(mtmp);
+
         if (offer >= demand) {
             verbalize("Very well, mortal. I shall not impede thy quest.");
             pline("%s vanishes, laughing about cowardly mortals.",