fileset: assertion failure in fs_reinit_merger on broken mtbl file
fileset: assertion failure in fs_reinit_merger on broken mtbl file

If there is a broken mtbl file in the fileset then a NULL reader will
be returned. This change checks for that error condition and does not
pass the empty reader onwards which causes an assertion failure in
mtbl_reader_source.

farsightsec/mtbl,farsightsec/mtbl