Bugfix: Race with first put/delete with a merge...

... where the biggest file in the keydir (name-wise) is 0 bytes,
e.g. created due to another merge race and was undone via un_write().

Failing test case:

    Cu2 = [[{set,{var,1},{call,bitcask_pulse,bc_open,[true]}},{set,{var,2},{call,bitcask_pulse,bc_close,[{var,1}]}},{set,{var,3},{call,bitcask_pulse,bc_open,[true]}},{set,{var,4},{call,bitcask_pulse,puts,[{var,3},{1,5},<<0,0,0>>]}},{set,{var,14},{call,bitcask_pulse,delete,[{var,3},1]}},{set,{var,15},{call,bitcask_pulse,bc_close,[{var,3}]}},{set,{var,16},{call,bitcask_pulse,bc_open,[true]}},{set,{var,18},{call,bitcask_pulse,puts,[{var,16},{2,6},<<0,0,0>>]}},{set,{var,21},{call,bitcask_pulse,merge,[{var,16}]}},{set,{var,22},{call,bitcask_pulse,delete,[{var,16},2]}},{set,{var,23},{call,bitcask_pulse,delete,[{var,16},2]}},{set,{var,24},{call,bitcask_pulse,fork_merge,[{var,16}]}},{set,{var,25},{call,bitcask_pulse,bc_close,[{var,16}]}},{set,{var,26},{call,bitcask_pulse,incr_clock,[]}},{set,{var,29},{call,bitcask_pulse,bc_open,[true]}},{set,{var,30},{call,bitcask_pulse,delete,[{var,29},1]}}],{1394,530114,962634},[{0,[]}]].
    eqc:check(bitcask_pulse:prop_pulse(), Cu2).

The last commands are: fork_merge, close, incr_clock, open, and delete key #1.

Error found:

    {exception,{'EXIT',{{badmatch,{error,{badmatch,{error,eexist}}}},
                        [{bitcask,do_put,6,[{file,"src/bitcask.erl"},{line,1297}]},
                         {bitcask,put,4,[{file,"src/bitcask.erl"},{line,267}]},
                         {bitcask,delete,2,[{file,"src/bitcask.erl"},{line,276}]},
                         {bitcask_pulse,delete,2,
                                        [{file,"test/bitcask_pulse.erl"},
                                         {line,857}]},
                         {eqc_statem,run_commands,2,
                                     [{file,"../src/eqc_statem.erl"},
                                      {line,737}]}]}}} /= ok