[xmlsec] clarification on openssl verification

[John Crowley]

xmlSecOpenSSLKeyDataX509VerifyAndExtractKey() checks keyInfoCtx- 
 >flags & XMLSEC_KEYINFO_FLAGS_X509DATA_STOP_ON_INVALID_CERT to  
determine whether to report error if part of a chain is missing.   
We're testing a self-signed cert and erroneously had something  
missing but xmlsec proceeded without error.

I can't find where xmlsec actually sets such a flag.

Thanks