rpmsg: glink: fix null pointer dereference on a null intent

In the case where glink->intentless is true and the call
to qcom_glink_tx fails then we have a condition where ret is
non-zero and intent is null, causing a null pointer deference
when setting intent->in_use to false.  Add an extra check to
only dereference intent if intent is non-null.

Detected by: CoverityScan CID#1455247 ("Explicit null dereferenced")

Fixes: 88c6060f ("rpmsg: glink: Handle remote rx done command")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>