Fix for BIO_gets and update documentation

[smittals2]

Description of changes:

BIO_gets takes a len/size parameter which is of type int and can be negative. Previously, this param was passed into callback_ex without a check for len being positive. This is an issue since callback_ex defines this size/len param as a size_t var. Passing in a negative number to size_t could lead to unexpected behavior.
A similar issue exists in BIO_read and BIO_write

This PR moves the check for len to before calling the callback and updates documentation for BIO_gets, BIO_read, and BIO_write.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.42%. Comparing base (697b277) to head (1ec6c34).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1756      +/-   ##
==========================================
- Coverage   78.44%   78.42%   -0.03%     
==========================================
  Files         580      580              
  Lines       96779    96779              
  Branches    13863    13863              
==========================================
- Hits        75920    75900      -20     
- Misses      20241    20260      +19     
- Partials      618      619       +1     

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