device-creator, stream-creator: Add a couple of assertions

Klocwork complained that source (in device-creator) and output (in
stream-creator) may be dereferenced if they're NULL. Let's make
Klocwork happy, and the code a bit more obvious to human readers as
well.

Change-Id: I835dd7d9da44e2866a97bc0424001a42c29602a8
main-volume-policy: Fix a memory leak

There can be multiple calls to pa_binding_target_info_new() in this
function, but only the last allocated info object was freed.

Change-Id: I9df43f0663b27b07ba7b8d01bc8ea9cc0a6c1b51