concurrent_hash_map: make double to size_t cast explicit in defragment()

[igchor]

and make sure only indexes up to max_index are used.

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This change is 

[lukaszstolarczuk]

Reviewed 1 of 1 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved

[codecov]

Codecov Report

Merging #860 into stable-1.9 will decrease coverage by 3.91%.
The diff coverage is 100.00%.

@@              Coverage Diff               @@
##           stable-1.9     #860      +/-   ##
==============================================
- Coverage       96.13%   92.22%   -3.92%     
==============================================
  Files              40       40              
  Lines            4144     3792     -352     
==============================================
- Hits             3984     3497     -487     
- Misses            160      295     +135     

Flag	Coverage Δ
#tests_clang_debug_cpp17	71.74% <0.00%> (-24.42%)	⬇️
#tests_gcc_debug	93.74% <100.00%> (+0.53%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ude/libpmemobj++/container/concurrent_hash_map.hpp	94.35% <100.00%> (-1.36%)	⬇️
include/libpmemobj++/detail/conversions.hpp	0.00% <0.00%> (-100.00%)	⬇️
include/libpmemobj++/timed_mutex.hpp	54.83% <0.00%> (-32.26%)	⬇️
include/libpmemobj++/pexceptions.hpp	33.33% <0.00%> (-20.91%)	⬇️
include/libpmemobj++/defrag.hpp	72.72% <0.00%> (-19.59%)	⬇️
include/libpmemobj++/detail/atomic_backoff.hpp	31.81% <0.00%> (-18.19%)	⬇️
include/libpmemobj++/shared_mutex.hpp	77.77% <0.00%> (-11.70%)	⬇️
include/libpmemobj++/make_persistent.hpp	80.76% <0.00%> (-11.54%)	⬇️
include/libpmemobj++/condition_variable.hpp	67.39% <0.00%> (-10.87%)	⬇️
include/libpmemobj++/container/vector.hpp	87.90% <0.00%> (-10.25%)	⬇️
... and 17 more

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[KFilipek]

Reviewed 1 of 1 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved

[szyrom]

Reviewable status: complete! all files reviewed, all discussions resolved