fix if u_value contains redundant data

[exiledkingcc]

fix #1288

the specification says:

To understand the algorithm below, it is necessary to treat the O and U strings in the Encrypt dictionary
as made up of three sections. The first 32 bytes are a hash value (explained below). The next 8 bytes are
called the Validation Salt. The final 8 bytes are called the Key Salt.

so /U and /O should be 48-bytes data.
but for the pdf which causes #1288 , /O 's length is 127-bytes, the redundant data are zeros.

[pubpub-zz]

great!

[codecov]

Codecov Report

Merging #1317 (5e17fb2) into main (eb0be4d) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1317   +/-   ##
=======================================
  Coverage   95.06%   95.06%           
=======================================
  Files          30       30           
  Lines        5044     5044           
  Branches     1041     1041           
=======================================
  Hits         4795     4795           
  Misses        141      141           
  Partials      108      108           

Impacted Files	Coverage Δ
PyPDF2/_encryption.py	92.62% <100.00%> (ø)

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[MartinThoma]

Very nice work @exiledkingcc ! Thank you - also for sharing the part of the specifications 🤗 This made it really easy to review / merge the PR