Spatializer: add missing `return`

It seems that we intended to return here. Otherwise, we'll deref NULL
below.

If68c99f39040984ff46e27779f211fdfe3126da5 adds attributes to these
functions that will have `clang` emit warnings if their results aren't
used.

Caught by the static analyzer:
> frameworks/av/services/audiopolicy/service/Spatializer.cpp:558:13:
warning: Dereference of null pointer (loaded from variable 'output')
[clang-analyzer-core.NullDereference]

Bug: 188502620
Test: TreeHugger
Change-Id: Idced7c095b6617349737449cf5451abdb22ea6ed