codec_speex: Fix trashing normal static frame for AST_FRAME_CNG.

Made use a local static frame to generate the AST_FRAME_CNG frame when
silence starts.

I don't think the handling of the AST_FRAME_CNG has ever really worked
because there doesn't seem to be any consumers of it.

Review: https://reviewboard.asterisk.org/r/3813/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@419206 65c4cc65-6c06-0410-ace0-fbb531ad65f3