staging: pi433: fix potential null dereference

[ Upstream commit 64c4c4ca6c129a4191e8e1e91b2d5d9b8d08c518 ]

Add a test for successful call to cdev_alloc() to avoid
potential null dereference. Issue reported by smatch.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Fixes: 874bcba6 ("staging: pi433: New driver")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>