Reset g_mgr_client when client is destroyed

Reset g_mgr_client when client is destroyed

- Issue:
App can access g_mgr_client even if vc mgr was deinitialized.

- Solution:
VC mgr client module did not assign null in g_mgr_client when
vc_mgr_client_destroy() is invoked. Thus, app still can access the
dereferenced pointer in g_mgr_client after calling
vc_mgr_deinitialized(). This patch sets null in g_mgr_client when app
invokes the vc_mgr_deinitialize(). Through this patch, the client
structure of vc mgr is properly reset.

Change-Id: I8c20e70960607257fa5e80b961c610b5aaf51f9a
Signed-off-by: Suyeon Hwang <stom.hwang@samsung.com>