Fixes issues of unsafe memory access

[fpdotmonkey]

This adds the tests test_raises_error_indexing_empty_buffer and test_negative_indices_give_recent_data_with_unfull_buffer to ensure the class doesn't access memory unsafely.

Closes #13 Closes #14

[codecov-commenter]

Codecov Report

Merging #15 into master will increase coverage by 0.07%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #15      +/-   ##
==========================================
+ Coverage   98.49%   98.57%   +0.07%     
==========================================
  Files           3        3              
  Lines         266      280      +14     
==========================================
+ Hits          262      276      +14     
  Misses          4        4              

Impacted Files	Coverage Δ
numpy_ringbuffer/__init__.py	98.33% <100.00%> (+0.05%)	:arrow_up:
tests.py	98.74% <100.00%> (+0.08%)	:arrow_up:

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