Don't interrupt tin opening if being slimed and tin is chameleon

Another SliceHack feature. It's possible for you to eat the chameleon
tin and turn into a fiery monster that burns off the slime in its
natural form, either extremely luckily at random or if you have
polymorph control.

src/eat.c

@@ -3357,8 +3357,10 @@ int threat;
     case STONED:
         return (boolean) (mndx >= LOW_PM
                           && (mndx == PM_LIZARD || acidic(&mons[mndx])));
-    /* no tins can cure these (yet?) */
+    /* polymorph into a fiery monster */
     case SLIMED:
+        return (boolean) (mndx == PM_CHAMELEON);
+    /* no tins can cure these (yet?) */
     case SICK:
     case VOMITING:
         break;