func_jitterbuffer: fix frame leaks.

Fix code paths where it is possible for frames to leak.
Fix uninitialized variable in jb_get_fixed and jb_get_adaptive.

ASTERISK-22409 #related
Reported by: Corey Farrell
Review: https://reviewboard.asterisk.org/r/4128/
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Merged revisions 427019 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 427020 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 427021 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@427022 65c4cc65-6c06-0410-ace0-fbb531ad65f3