net: qualcomm: rmnet: check for null ep to avoid null pointer dereference

The call to rmnet_get_endpoint can potentially return NULL so check
for this to avoid any subsequent null pointer dereferences on a NULL
ep.

Detected by CoverityScan, CID#1465385 ("Dereference null return value")

CRs-Fixed: 2156182
Change-Id: I2119f695279b0f1f65e5848f00193a5e3d8f3063
Fixes: 23790ef12082 ("net: qualcomm: rmnet: Allow to configure flags for existing devices")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Git-commit: 0c29ba1b43df1eb7d8beb03fc929d2dac4c15f7e
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>