soc: hab: Fix a memory leakage when unexport

When the remote vchan is closed and unexport happens
in local at the same time, function hab_get_vchan_fromvcid
will return null in hab unexport, which will cause memory leak.

Change-Id: I8dac4f4154f24734dc2a11aa73f726cd705cc291
Signed-off-by: Yajun Li <yajunl@codeaurora.org>