commit 50010c20 upstream.

This is decrementing the pointer, instead of the value stored in the
pointer.  KASan detects it as an out of bounds reference.
Reported-by: "Berry Cheng 程君(成淼)" <chengmiao.cj@alibaba-inc.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>