memshare: Free QMI handle only if its valid

Avoid possible use-after-free access for a QMI handle
during driver deinit by ensuring the free happens only for
valid QMI handle and is marked NULL post free.

Change-Id: I4314dff560de2fc3aea30d636f935d7a02067a57
Signed-off-by: Manoj Prabhu B <quic_bmanoj@quicinc.com>