Fix memory leaks in Windows TLS code

[graebm]

Issue:

A C++ SDK user found a memory leak in the Windows TLS channel-handler code and submitted the following PR: #651

In evaluating this 2-line fix, I noticed that it sat in the middle of some (preexisting) sloppy error-handling.

Description of changes:

• Fix memory leak, as shown in @normanade's PR, by calling FreeContextBuffer().
  • Change it so we always call FreeContextBuffer() after calling to AcceptSecurityContext()/InitializeSecurityContextA(). Previously we'd only free the buffer in certain circumstances, and were probably leaking.
• Remove (often sloppy) error-handling that tries to deal with allocation failures of aws_io_message. Officially declare that message allocations can't fail.
  • NOTE: Ever since this change in 2021, our allocators will never return NULL. They deal with OOM by immediately terminating the program. This was done because error-handling code for allocation failure makes everything so much more complex, and it's seldom tested. We tried for years to handle it. It's not worth the effort.
• Fix some error-handling that forgot to delete the aws_io_message after checking its capacity
• In the Windows TLS code, replace some AWS_ASSERT() about buffers being large enough with AWS_FATAL_ASSERT(). Similarly, add an AWS_FATAL_ASSERT() in one place that never checked or asserted before doing the copy.
  • I don't know enough about TLS or Windows APIs to say whether or not these deserve real error-handling. So I'm just preserving the status quo (but crashing instead of doing undefined behavior).

Co-authored-by: @normanade

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

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 80.22%. Comparing base (9072f86) to head (5720360).

Files	Patch %	Lines
source/channel.c	66.66%	1 Missing ⚠️
source/s2n/s2n_tls_channel_handler.c	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #652      +/-   ##
==========================================
+ Coverage   80.12%   80.22%   +0.10%     
==========================================
  Files          28       28              
  Lines        5967     5957      -10     
==========================================
- Hits         4781     4779       -2     
+ Misses       1186     1178       -8     

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