Added check if serialized string contains an object to prevent PHP object injection

If the serialized string contains an object code execution is terminated to avoid PHP object injection.

This vulnerability affects the following application versions:

• All in One SEO Pack 4.0.15
• All in One SEO Pack 4.0.16
• All in One SEO Pack 4.0.17
• All in One SEO Pack 4.0.18
• All in One SEO Pack 4.1.0.1