slimbus: avoid null pointer dereference on msg

The pointer msg is checked to see if it is null at the start of
the function and jumps to the error exit label reterr that then
dereferences msg when it prints a dev_err error message. Avoid
this potential null pointer dereference by only printing the
error message if msg is not null.

Detected by CoverityScan, CID#1463141 ("Dereference after null check")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>