Fix bug with reading in mappings for disk_index.
Fix bug with reading in mappings for disk_index.

Symptom was mmap_file exceptions when trying to use an existing index.

saq7/MeTA,esparza83/meta,husseinhazimeh/meta,esparza83/meta,esparza83/meta,esparza83/meta,gef756/meta,gef756/meta,husseinhazimeh/meta,husseinhazimeh/meta,gef756/meta,gef756/meta,gef756/meta,saq7/MeTA,saq7/MeTA,husseinhazimeh/meta,husseinhazimeh/meta,esparza83/meta