Fix potential large memory allocation in DerInputStream

[nhartner]

The object length limit check was adding 2 int values which could result in an overflow exception, allowing very large lengths to bypass the limit enforcement. Now the limit check casts the value to a long before doing addition to avoid integer overflow.

[sappenin]

LGTM

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.77%. Comparing base (f50ad63) to head (c79b2cf).

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #43      +/-   ##
============================================
+ Coverage     82.32%   82.77%   +0.44%     
- Complexity      174      176       +2     
============================================
  Files            33       33              
  Lines          1126     1126              
  Branches         99       99              
============================================
+ Hits            927      932       +5     
+ Misses          153      149       -4     
+ Partials         46       45       -1     

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