Always check lib.sftp_dir_eof() in readdir()

[JamesWrigley]

lib.sftp_readdir() can return null if there was an error or if it's at the end of the directory.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 63.04%. Comparing base (cf8fc39) to head (8c7d5d7).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/sftp.jl	66.66%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
+ Coverage   62.97%   63.04%   +0.06%     
==========================================
  Files          11       11              
  Lines        3001     3006       +5     
==========================================
+ Hits         1890     1895       +5     
  Misses       1111     1111              

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