Fix memtracer bad assumptions on the size of stack trace

[DmitriyMusatkin]

Issue #, if available:

Description of changes:
Fix memtracer assumptions on the size of stack. With optimizations on, there is no guarantees how many frames we get back. So handle any size of stack gracefully.

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 72.72727% with 3 lines in your changes missing coverage. Please review.

Project coverage is 83.08%. Comparing base (06cf4d8) to head (e270eed).

Files	Patch %	Lines
source/memtrace.c	72.72%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1122      +/-   ##
==========================================
- Coverage   83.12%   83.08%   -0.05%     
==========================================
  Files          56       56              
  Lines        5755     5758       +3     
==========================================
  Hits         4784     4784              
- Misses        971      974       +3     

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