fix github issue #302 - divide by 0 crash

fix github issue #302 - divide by 0 crash

The traceback points directly to the problem:  divide by 0 happens
if the 'bogusmon' file only contains the "do not edit" line, which
would happen if 'bogusmon.txt' is empty.  makedefs probably ought to
complain about that.

There is now one hardcoded bogus monster to fall back to:  'bogon'.

Random tombstone epitaphs report divide by 0 if their text source is
empty, but it is done by rn2() rather than rn2_for_display_rng() so
is just a warning for pre-release code.  It would crash for release
version though.

I tried placing an empty engravings file and expected similar results
but didn't see any response.  Not sure what that means.

After the fix, empty epitaph file yields blank result so graves that
want a random epitaph won't have any epitaph.

Fixes #302