FIx undefined behavior in dirvote.c

 Fix a bug in the voting algorithm that could yield incorrect results
 when a non-naming authority declared too many flags. Fixes bug 9200;
 bugfix on 0.2.0.3-alpha.

Found by coverity scan.
Add a comment and a check for why flag indices will be <= 63
Merge branch 'bug9200' into maint-0.2.4
Merge remote-tracking branch 'origin/maint-0.2.4'