merger: merger_iter_add_entry(): handle case where iterator is NULL after initial entry_fill()
merger: merger_iter_add_entry(): handle case where iterator is NULL after initial entry_fill()

snarkmaster/mtbl,snarkmaster/mtbl,rep/mtbl,snarkmaster/mtbl,farsightsec/mtbl,rep/mtbl,farsightsec/mtbl,rep/mtbl