slimbus: slim-msm-ngd: Fix corner case to avoid NULL access

For asynchronus read transfer, BAM RX callback can be received
before main thread completes ngd_xfer_msg function execution.

Keep a check to avoid accessing transfer structure, which
may get freed in early RX callback.

Signed-off-by: Shrey Vijay <shreyv@codeaurora.org>
Change-Id: Ie96e669be9282687acd22365b68ac9f4ae659030