alphabet sour warning

alphabet sour warning

A recently added impossible to check for an(Null) and an("") was
triggered by the fuzzer:  Alphabet soup: 'an("")'.  I reproduced it a
couple of times and tracked it do_screen_description(for '/' command)
matching the symbol from mapglyph to monster class #0, a placeholder
with symbol value '\0'.  So mapglyph() returned a symbol of '\0', but
not necessary from showsyms[0 + SYM_OFF_M].

The pager lookup code's monster loop shouldn't have been attempting
to match against class #0, and since this fix I haven't been able to
reproduce the situation again.  But I also didn't trigger it with a
bunch of temporary checks in mapglyph() so don't know what is really
going on under the hood.