xdp: add NULL pointer check in __xdp_return()

rhashtable_lookup() can return NULL. so that NULL pointer
check routine should be added.

Fixes: 02b55e56

 ("xdp: add MEM_TYPE_ZERO_COPY")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>