soc: qcom: mem-buf: Fix NULL pointer dereference when assigning memory

The xfer_mem pointer is dereferenced before sanitizing it, so use it
only after it has been sanitized.

Change-Id: Iaedd988903f012e647dcfee9bbe7905e0a4574eb
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>