bin/media_client: Fix memory leak

Fix leak of memory allocated to 'app'.
Pointed out by Clang Static Analyzer.

Change-Id: I59d39b76a18893bd284db1d75e40620bb1aee9e2
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3480
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>
Reviewed-by: Axel Dörfler <axeld@pinc-software.de>

media_client: print usage in case of missing arguments

Now the obvious "media_client somefile.wav" will tell you what to do.

Move media_client to bin