soc: qcom: hab: add rx queue empty check after getting the lock

Need to do empty check again when we have the lock of the rx queue of
a given virtual channel. Without this check, it is possible that the
list head of the queue will be wrongly taken as a valid message node
if the valid message has already been fetched by some other thread.

Change-Id: I65f73acd86fe2fb709e2af2ffeaa42581b4168ca
Signed-off-by: Yong Ding <yongding@codeaurora.org>