soc: qcom: hab: initialize the hab_message pointer as NULL

The had_message pointer should be initialized as NULL before
calling hab_vchan_recv(). Furtherly, inside the hab_vchan_recv(),
it is good to also clear it as NULL when the virtual channel
is not found. Then there is such guarantee by both the caller
and the callee.

Change-Id: Id8c46e1828fb3af0c42a9f59f68148717c511e6e
Signed-off-by: Yong Ding <yongding@codeaurora.org>