Prevent accessing outside the mons array

Fix another out-of-bounds error
Monster trying to decide if a tin could cure stoning looking at
a special or empty tin would do acidic(&mons[-1])

doc/fixes36.5

@@ -7,6 +7,8 @@ produce 3.6.5 as well as any post-release fixes in binaries.
 General Fixes and Modified Features
 -----------------------------------
 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
 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()