Fix blockstream decryption bug causing signature verification failures

[covert-encryption]

• The blockstream pos variable was not updated correctly in some special cases, in particular with files slightly above 1024 bytes.
• Added test for such case.
• Removed unused filelen and filepos variables.
• Don't guess initial nextlen, use block0's known size.

Fix #79

[codecov[bot]]

Codecov Report

Merging #80 (297f41d) into main (76a7570) will increase coverage by 0.49%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #80      +/-   ##
==========================================
+ Coverage   74.47%   74.96%   +0.49%     
==========================================
  Files          23       23              
  Lines        2174     2169       -5     
  Branches      510      509       -1     
==========================================
+ Hits         1619     1626       +7     
+ Misses        432      422      -10     
+ Partials      123      121       -2     

Impacted Files	Coverage Δ
covert/blockstream.py	87.04% <100.00%> (+5.73%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 76a7570...297f41d. Read the comment docs.

[covert-encryption]

Should be good now.