Fix NetFlow V9 loop with unreachable exit condition (7.1)

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Note: This is a backport of #27127 to 7.1.

Description

Fix NetFlow V9 loop with unreachable exit condition.

Motivation and Context

A crafted NetFlow V9 packet could cause the parser to loop indefinitely.

closes https://github.com/Graylog2/graylog-plugin-enterprise/issues/15330

How Has This Been Tested?

Added regression tests in NetFlowV9ParserTest.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have requested a documentation update.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.