Fix warning and empty hash

[fredeom]

Sometimes stream is read even after reaching eof. In this case hash_final is called on obsolete hashResource producing warning and destroys hash result. Checking that hash is null prevents this behavior. User still can seek(0) and set hash to null in initializeHash function.

Use case: AesDecryptingStream:113 calls read on empty stream (in our case its HashingStream) in order to prefetch data. Results:

• Warning is produced.
• Hash is spoiled.

[codecov[bot]]

Codecov Report

Merging #18 (5acd6d8) into master (0cd5b6a) will not change coverage. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##              master       #18   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity       113       114    +1     
===========================================
  Files             12        12           
  Lines            300       301    +1     
===========================================
+ Hits             300       301    +1     

Impacted Files	Coverage Δ	Complexity Δ
src/AesDecryptingStream.php	100.00% <100.00%> (ø)	21.00 <0.00> (ø)
src/AesEncryptingStream.php	100.00% <100.00%> (ø)	18.00 <0.00> (ø)
src/AesGcmDecryptingStream.php	100.00% <100.00%> (ø)	4.00 <0.00> (ø)
src/AesGcmEncryptingStream.php	100.00% <100.00%> (ø)	5.00 <0.00> (ø)
src/HashingStream.php	100.00% <100.00%> (ø)	15.00 <0.00> (+1.00)

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