Fix bunch of minor oss-fuzz/coverity issues.

[ni4]

As title says.

[codecov]

Codecov Report

Attention: Patch coverage is 96.29630% with 1 line in your changes missing coverage. Please review.

Project coverage is 84.67%. Comparing base (a421aa3) to head (064c77a).
Report is 22 commits behind head on main.

Files with missing lines	Patch %	Lines
src/librepgp/stream-sig.cpp	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2285      +/-   ##
==========================================
+ Coverage   84.65%   84.67%   +0.01%     
==========================================
  Files         116      116              
  Lines       23422    23432      +10     
==========================================
+ Hits        19829    19840      +11     
+ Misses       3593     3592       -1     

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[maxirmx]

LGTM

[ni4]

@ronaldtse @desvxx Ping for review!

[ni4]

Thanks, merging with two approvals.