prima: check for NULL pointer before accessing roc context

Bug: 20500889

Remain on channel context can be NULL in __wlan_hdd_mgmt_tx if
roc is not running already. So check for NULL before accessing
the roc context.

Change-Id: If1f7a1cf6b372882a078f54d0615d5a9bf7dc3ca
Signed-off-by: Bulbul Dabi <bdabi@codeaurora.org>