onelogin/php-saml Improper signature validation on LogoutRequest/LogoutResponse.

In order to verify Signatures on Logoutrequests and LogoutResponses we use
the verifySignature of the class XMLSecurityKey from the xmlseclibs library.
That method end up calling openssl_verify() depending on the signature algorithm used.

The openssl_verify() function returns 1 when the signature was successfully verified,
0 if it failed to verify with the given key, and -1 in case an error occurs.
PHP allows translating numerical values to boolean implicitly, with the following correspondences:

• 0 equals false.
• Non-zero equals true.

This means that an implicit conversion to boolean of the values returned by openssl_verify()
will convert an error state, signaled by the value -1, to a successful verification of the
signature (represented by the boolean true).

The LogoutRequest/LogoutResponse signature validator was performing an implicit conversion to boolean
of the values returned by the verify() method, which subsequently will return the same output
as openssl_verify() under most circumstances.
This means an error during signature verification is treated as a successful verification by the method.

Since the signature validation of SAMLResponses were not affected, the impact of this security
vulnerability is lower, but an update of the php-saml toolkit is recommended.