same as in other places, check that generator->release is not NULL

before calling it.
This allows generators to set it to NULL when they have nothing to
do there.

Later, the three copies of the code that releases a generator
should be moved to a function.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@48766 65c4cc65-6c06-0410-ace0-fbb531ad65f3