USB: f_rndis: Fix NULL pointer dereference during composition switch

During fast RNDIS enable/disable, there is a chance that rndis
driver unbind race with rndis_cmd_complete and results in NULL
pointer dereference. Fix this by having check it before handling
rndis command received.

Change-Id: Ie54c5083866db9f9d1b80bb5438b1d82db15580f
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>
Signed-off-by: Sivasri Kumar Vanka <sivasri@codeaurora.org>