RFCOMM: Prevent sending data in BTA_JV_ST_NONE state

This patch adds state check in bta_jv_rfcomm_write, to prevent sending
data in BTA_JV_ST_NONE state. This would prevent an ASSERT in the code
due to a null handle. The handle might have already been set to null
during the closing of the RFCOMM connection.

Change-Id: Ibc6b1fb7aa6dce9a4acb52d724af3023fad9a9ae