_hashindex.c: more fixes

[ThomasWaldmann]

• avoid buckets_length integer overflow on 32bit systems via huge num_buckets
• always initialize index-> min_empty and num_empty
• correctly free memory when header validation fails. this is a minor issue, because borg will terminate in that case anyway.
• make it possible to lookup in compacted hashtables
• deal safely with empty index: we must use num_buckets = 1 to avoid division by zero and sanity check in hashindex_read.
• reinitialize upper/lower limit and min_empty after compact
• fix size_idx / fit_size / grow_size / shrink_size (mind array bounds)
• deal with growing when already at max capacity
• hashindex_resize: replace num_entries assertion, rather return error

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.96%. Comparing base (7fd6df2) to head (eb11772).
⚠️ Report is 1 commits behind head on 1.4-maint.

Additional details and impacted files

@@              Coverage Diff              @@
##           1.4-maint    #9575      +/-   ##
=============================================
- Coverage      81.97%   81.96%   -0.01%     
=============================================
  Files             38       38              
  Lines          11299    11299              
  Branches        1778     1778              
=============================================
- Hits            9262     9261       -1     
- Misses          1452     1453       +1     
  Partials         585      585              

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