fix: map unmapped reads (tid=-1) to correct index in slow_idxstats

[nh13]

When iterating CRAM records, unmapped reads have tid == -1. Casting
this to usize wraps to usize::MAX, causing an index-out-of-bounds
panic. Map tid == -1 to the designated unmapped slot at index nref.

[coveralls]

Pull Request Test Coverage Report for Build 23730666310

Details

• 7 of 7 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 81.81%

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Totals
Change from base Build 21474746563:	0.03%
Covered Lines:	2793
Relevant Lines:	3414

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💛 - Coveralls