linux-user: Check lock_user() return value for NULL

lock_user() can return NULL, which typically means the syscall
should fail with EFAULT. Add checks in various places where
Coverity spotted that we were missing them.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
