Fail PORT_GetChannelInfo when p_mcb is null

There is a case where the multiplexer is shut down and PORT_CLOSED is
processed before BTA_JV_RFCOMM_CLOSE_EVT.  In this case,
PORT_GetChannelInfo should fail to prevent null pointer dereference.

Bug: 400636306
Test: m com.android.bt
Flag: EXEMPT adding null check and log
Change-Id: If6efb16cadc28a710465046886a9f6680fa95a8c