pml/ob1: Fix segmentation fault on CUDA path.

Fix segfault due to mca_pml_ob1_cuda_need_buffers not handling the case of the
endpoint not being there. Calling mca_bml_get_endpoint() seems to fix the problem.

Fixes open-mpi/ompi#1402

ompi/mca/pml/ob1/pml_ob1_cuda.c

@@ -156,8 +156,7 @@ int mca_pml_ob1_cuda_need_buffers(void * rreq,
                                   mca_btl_base_module_t* btl)
 {
     mca_pml_ob1_recv_request_t* recvreq = (mca_pml_ob1_recv_request_t*)rreq;
-    mca_bml_base_endpoint_t* bml_endpoint =
-        (mca_bml_base_endpoint_t*)recvreq->req_recv.req_base.req_proc->proc_endpoints[OMPI_PROC_ENDPOINT_TAG_BML];
+    mca_bml_base_endpoint_t* bml_endpoint = mca_bml_base_get_endpoint (recvreq->req_recv.req_base.req_proc);
     mca_bml_base_btl_t *bml_btl = mca_bml_base_btl_array_find(&bml_endpoint->btl_send, btl);
 
     /* A btl could be in the rdma list but not in the send list so check there also */