Add an fai_line_length function.

[jkbonfield]

This exposes the input datas line wrap length (minus white-space / newlines).

Also fixed a SEGV in fai_retrieve given incorrect or malicious .fai files with a zero length "blen" figure.

[daviesrob]

Looks OK, although I spotted a couple of TODOs for later.

• fai_get_val() fbeg and fend are hts_pos_t *, but everything calling it passes int64_t *. This affects more then the new function, so probably best tidied up separately.
• Currently, zero-length sequences get excluded from the index, but if they were present they would presumably have all lengths as zero and would upset the new blen check. I think there may be other issues around including such horrors, so I don't think it's a problem at the moment.