CryptoPkg BaseCryptLib: Init the content of struct 'CertCtx' before use

CryptoPkg BaseCryptLib: Init the content of struct 'CertCtx' before use

Some fields in structure 'CertCtx' might be used uninitialized in function
Pkcs7GetCertificatesList().

This commit makes sure that 'CertCtx' gets initialized before being used.

Cc: Long Qin <qin.long@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Qin Long <qin.long@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>