Avoid compiler warning

[justsmth]

Issues:

Addresses aws/aws-lc-rs#574

Context

The compiler cannot always confirm that the array indices used are within the required bounds. On certain platform, specifically powerpc64le in this case, the compiler may generate a warning about it. For our build, warnings are treated as errors thus causing compilation to fail.

Description of changes:

Adds sanity checks to several locations in gcm.c that abort if the logic would reference an out-of-bounds index of an array.

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 50.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 78.77%. Comparing base (c9d48a6) to head (c64f830).
Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
crypto/fipsmodule/modes/gcm.c	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1981      +/-   ##
==========================================
- Coverage   78.79%   78.77%   -0.03%     
==========================================
  Files         590      590              
  Lines      101506   101512       +6     
  Branches    14401    14403       +2     
==========================================
- Hits        79979    79962      -17     
- Misses      20891    20915      +24     
+ Partials      636      635       -1     

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