General Fixes and Modified Features
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-have string_for_opt() return empty_optstr on failure
fix accessing mons[-1] when trying to gate in a non-valid demon
fix accessing mons[-1] when monster figures out if a tin cures stoning
+have string_for_opt() return empty_optstr on failure
ensure existing callers of string_for_opt() check return value before using it
fix potential buffer overflow in add_menu_coloring()
fix potential buffer overflow in sym_val()