Slim-msm: Add NULL pointer checks

Don't access the qmi pointers in qmi_exit function
when the qmi_init is failed, as these pointers are
freed in qmi_init if any error occured.

Change-Id: I5c13fe7b5ba08bf29329aad4a6cbf6fc4601260a
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>