volume listener: check for correct effect descriptor when releasing

It is possible that the effect descriptor in the listener
context is NULL. Attempting to access its fields would result
in a SIGSEGV. Add a check to avoid crashing.

Bug: 63780779
Test: HAL should not be crashing in vol_prc_lib_release
Change-Id: I241890840acab56e0c706ffca542fa2b1ffef02a