btl/openib: fix segmentation fault

This commit fixes a segmentation fault that occurs if a device can be
initialized but not used. In this case the devices_count is not equal
to the number of usable devices in the devices pointer array.

Thanks to @artpol84 for tracking this down.

Fixes open-mpi/ompi#1823

Signed-off-by: Nathan Hjelm <hjelmn@lanl.gov>

opal/mca/btl/openib/btl_openib_component.c

@@ -3771,8 +3771,10 @@ static int btl_openib_component_progress(void)
     for(i = 0; i < mca_btl_openib_component.devices_count; i++) {
         mca_btl_openib_device_t *device =
             (mca_btl_openib_device_t *) opal_pointer_array_get_item(&mca_btl_openib_component.devices, i);
+        if (NULL != device) {
             count += progress_one_device(device);
         }
+    }
 
 #if OPAL_CUDA_SUPPORT /* CUDA_ASYNC_SEND */
     /* Check to see if there are any outstanding dtoh CUDA events that