Fix pem validation

[DmitriyMusatkin]

Issue #, if available:

Description of changes:
Pem validation logic had a potential to read past the bound to check for header when input terminated prematurely with a dash

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.69%. Comparing base (dfbf8aa) to head (ec5f12d).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #735      +/-   ##
==========================================
- Coverage   78.74%   78.69%   -0.06%     
==========================================
  Files          30       30              
  Lines        6385     6370      -15     
==========================================
- Hits         5028     5013      -15     
  Misses       1357     1357              

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[TingDaoK]

fix&ship