Add check for incorrect length values in postgres DataRow parser

[fearful-symmetry]

Proposed commit message

This fixes a bug where a PostgreSQL packed with a DataRow that specified a row count less than the actual count of rows would result in an out of bounds array access and panic. This just adds a length check, and a bit of error cleanup.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

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[mergify]

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[elasticmachine]

Pinging @elastic/sec-linux-platform (Team:Security-Linux Platform)