net: qualcomm: rmnet: Fix possible null dereference in command processing

If a command packet with invalid mux id is received, the packet would
not have a valid endpoint. This invalid endpoint maybe dereferenced.
Identified by manual code inspection.

CRs-Fixed: 2156182
Change-Id: I2fb1cf159813e7c2fd1240a16411a654cb67d7d7
Fixes: 3352e6c45760 ("net: qualcomm: rmnet: Convert the muxed endpoint to hlist")
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>