SAML2/SignedElementHelper: Fix getValidatingCertificates.

This function did not properly validate signatures containing multiple
certificates (as is typical for metadata with certificate chains. The
fix is to ignore exceptions during validation.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2461 44740490-163a-0410-bde0-09ae8108e29a