Squiz/DisallowMultipleAssignments: fix false positive when handling parameter default values

[rodrigoprimo]

Description

This PR fixes a bug in the code used to check if a equal sign is used to set a default value for a parameter. This bug caused a false positive when the function declaration was missing the closing parenthesis (which can happen during live coding).

Suggested changelog entry

Squiz.PHP.DisallowMultipleAssignments false positive for parameter default values when the closing parenthesis of the function declaration is missing.

Related issues/external references

Fixes https://github.com/PHPCSStandards/PHP_CodeSniffer/issues/551

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
  • [ ] This change is only breaking for integrators, not for external standards or end-users.
• [ ] Documentation improvement

PR checklist

• [x] I have checked there is no other PR open for the same change.
• [x] I have read the Contribution Guidelines.
• [x] I grant the project the right to include and distribute the code under the BSD-3-Clause license (and I have the right to grant these rights).
• [x] I have added tests to cover my changes.
• [x] I have verified that the code complies with the projects coding standards.
• [ ] [Required for new sniffs] I have added XML documentation for the sniff.