nvmf/rdma: fix missing return statement

In case of failure during resource allocation within poll_group_create
there is a lack of return statement which could lead to NULL ptr
dereference.

Signed-off-by: Jacek Kalwas <jacek.kalwas@intel.com>
Change-Id: I84abe64a1843117d76b97e62656bdfc4fe2b35d8
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/463195
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Seth Howell <seth.howell@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>

lib/nvmf/rdma.c

@@ -3052,6 +3052,7 @@ spdk_nvmf_rdma_poll_group_create(struct spdk_nvmf_transport *transport)
 				SPDK_ERRLOG("Unable to allocate resources for shared receive queue.\n");
 				spdk_nvmf_rdma_poll_group_destroy(&rgroup->group);
 				pthread_mutex_unlock(&rtransport->lock);
+				return NULL;
 			}
 		}