thawing ice followup

Modifying an() [actually just_an()] to treat "<thickness> ice" and
"frozen <hallucinatory liquid>" as special cases which shouldn't be
prefixed with "a" or "an" affected using something like "shaved ice"
or "frozen yogurt" as named fruit.
 |a) shaved ice
 |b) frozen yogurt (weapon in hand)
now have article "a" preceding them:
 |a) a shaved ice
 |b) a frozen yogurt (weapon in hand)
However, the existing cases
 |c) iron bars
 |d) an iron bars (weapon in hand)
still get item 'c' wrong.  'd' is slightly odd but that's because the
fruit name is ambiguous as to whether it's singular or plural.

src/pickup.c

@@ -365,7 +365,7 @@ describe_decor(void)
     } else if (dfeature) {
         if (waterhere)
             dfeature = strcpy(fbuf, waterbody_name(u.ux, u.uy));
-        if (strcmp(dfeature, "swamp"))
+        if (strcmp(dfeature, "swamp") && ltyp != ICE)
             dfeature = an(dfeature);
 
         if (Verbose(2, describe_decor1)) {