Fix for C343-235 (protection spell grammar) checks wrong location

[tung]

Fix the fix for C343-235

C343-235: "Casting spell of protection gives incorrect message if hero is swallowed or in rock."

This corrects the check to use "stone" versus "air" when embedded in stone to use the player's position, otherwise, most of the time protection is cast the message will mention stone instead of air.

From UnNetHack r794, to go with https://github.com/DanielT/NitroHack/commit/adce07daf548763a77388ea690ac76da950a1834.

Working copy of this fix at https://github.com/tung/NitroHack/commit/02e6a13e6f47a57ed4f057298a9b18df99394f3f.