IB/mad: Remove redundant NULL pointer check in ib_mad_recv_done_handler()

In ib_mad_recv_done_handler(), the response pointer is checked for
NULL after allocating it.  It is then checked again in the local
process_mad() path but there is no possibility of it changing in
between.

Signed-off-by: Ralph Campbell <ralph.campbell@qlogic.com>
Acked-by: Hal Rosenstock <hal@xsigo.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>