ofproto-dpif: Make ofport_dpif_cast() internally consistent.

ofproto-dpif: Make ofport_dpif_cast() internally consistent.

This function's assertion would dereference a null pointer given a null
'ofport' argument, but its return statement checked for a null pointer
argument.  This commit fixes the inconsistency in favor of supporting
null pointer arguments.  (I discovered this problem while writing a piece
of code that wanted support for a null pointer argument, otherwise I would
resolve the inconsistency in the other direction.)

Signed-off-by: Ben Pfaff <blp@nicira.com>