opusenc: fix crash when setting "cbr" property when encoder is not running yet

https://bugzilla.gnome.org/show_bug.cgi?id=692698
Automatic update of common submodule

From a942293 to 2de221c
autogen.sh: allow calling from out-of-tree

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>

https://bugzilla.gnome.org/show_bug.cgi?id=692859