net: netfilter: Updating conntract pointer NULL check

The conntrack (ct) NULL check was done after we have accessed the
ct pointer. Moving the NULL check before accessing any ct related
parameters.

Change-Id: I8334728747c35ed21a8c0420add577bc3135a3bd
Signed-off-by: Sridhar Ancha <sancha@codeaurora.org>