net: rmnet_data: Check for endpoint validity when demuxing

In case an invalid mux id is passed, an invalid device lookup is
done leading to passing on the packet to stack incorrectly rather
than consuming them.

CRs-Fixed: 2181180
Change-Id: Iad905e932b49eda00a7d91e906f0ac0ca44cdb59
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>