format_ogg_speex: Implement a "not supported" write handler

This format did not specify a "write" handler, so when attempting to write
to it (ast_writestream) a crash would occur.

This patch adds a default handler that simply issues a "not supported"
warning, thus no longer crashing.

ASTERISK-29539

Change-Id: I8f6ddc7cc3b15da30803be3b1cf68e2ba0fbce91