netfilter: contrack: Null pointer check is added

Prior to null pointer check, SIP node was deleted from the list
a null pointer check is added to confront the exception.

Change-Id: Ia12fa468eed7d1a91fad96840fa27cb0e4e208a8
Acked-by: Vinisha Varre <vvarre@qti.qualcomm.com>
Signed-off-by: Paras Singh Jain <parassin@codeaurora.org>