Avoid processing null elements of an array in Vulnerability Detector

[crolopez]

Related issue
#3456

Description

This PR prevents the bug described in the issue, and also protects it from happening in the other string array that we can find in a vulnerability node: affected_packages.

However, a null element in affected_packages does not produce a segmentation fault as it does if this element is found in advisories.

The fix consists of adding a null check on the elements of the array.

Configuration options

Considering that this error is not replicable with the currently hosted feed, the configuration must involve an offline update. We can use url or path for this.

    <feed name="redhat">
      <disabled>no</disabled>
      <path>/local_path/feed.json</path>
      <update_from_year>2010</update_from_year>
      <update_interval>1m</update_interval>
    </feed>

To replicate it, we have to download a feed and modify one of its vulnerabilities by adding null value to an array of strings like:

    "advisories": [
      "RHSA-2014:0429",
      "RHSA-2014:0452",
      "RHSA-2014:0401",
      "RHSA-2014:0253",
      "RHSA-2014:0252",
      "RHSA-2015:1009",
      "RHSA-2014:0525",
      "RHSA-2014:0526",
      "RHSA-2014:0527",
      "RHSA-2014:0528",
      "RHSA-2014:0400",
      null
    ]

Tests

• Configuration of null elements in the rest of the vulnerability variables (previously controlled).
• Compilation without warnings in every supported platform
  • Linux
• Source installation
• Package installation
• Source upgrade
• Package upgrade
• Memory tests
  • Valgrind report for affected components
  • CPU impact
  • RAM usage impact
• Retrocompatibility with older Wazuh versions
• Working on cluster environments