pngcrush: don't print out an error on mv if optimizing fails

pngcrush is currently storing the optimized png in <pngname>.out.
If optimizing the png fails, e.g. on palette errors, mv can't
overwrite the original file with the nonexisting .out file and prints
out an error like this:

mv: cannot stat `./foo/bar/baz.png.out': No such file or directory

Writing stderr to /dev/null will suppress that message.

Change-Id: I0b7339392545e98586d66cc8ff8abf52c443f66f