RDMA/amso1100: Add check for NULL reply_msg in c2_intr()

Fix a place where we might dereference a NULL pointer; this fixes
Coverity CID 1392.  On inspection I also found a place where we could
attempt to kmem_cache_free() a NULL pointer, so fix this too.

Signed-off-by: Tom Tucker <tom@opengridcomputing.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>