Fix i2d behavior for i2d_SSL_SESSION

[samuel40791765]

Issues:

Addresses CryptoAlg-2700

Description of changes:

i2d_SSL_SESSION wasn't exactly following the correct behavior of the legacy i2d functions when pp is non-NULL, but *pp is NULL. This caused issues with functions expecting a newly allocated buffer when calling i2d_SSL_SESSION with the recommended documented behavior. See i2d_SAMPLE for more details.

aws-lc/include/openssl/asn1.h

Lines 254 to 266 in 48a4057

	// If |outp| is non-NULL but |*outp| is NULL, the function sets |*outp| to a
	// newly-allocated buffer containing the result. The caller is responsible for
	// releasing |*outp| with |OPENSSL_free|. This mode is recommended for most
	// callers.
	//
	// If |outp| and |*outp| are non-NULL, the function writes the result to
	// |*outp|, which must have enough space available, and advances |*outp| just
	// past the output.
	//
	// WARNING: In the third mode, the function does not internally check output
	// bounds. Failing to correctly size the buffer will result in a potentially
	// exploitable memory error.
	int i2d_SAMPLE(const SAMPLE *in, uint8_t **outp);

Call-outs:

N/A

Testing:

New test below usage of i2d_SSL_SESSION to verify the behavior.

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

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.75%. Comparing base (48a4057) to head (86a2f57).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
ssl/ssl_asn1.cc	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1966      +/-   ##
==========================================
+ Coverage   78.73%   78.75%   +0.02%     
==========================================
  Files         590      590              
  Lines      101428   101476      +48     
  Branches    14384    14394      +10     
==========================================
+ Hits        79856    79916      +60     
+ Misses      20935    20924      -11     
+ Partials      637      636       -1     

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