util: Fix crash when dlopen() fails.
util: Fix crash when dlopen() fails.

imp_free() calls g_mutex_free() on imp->mutex, which is still NULL.

Signed-off-by: René Stadler <1d6e1cf70ec6f9ab28d3ea4b27a49a77654d370e@renestadler.de>

mrchapp/gst-openmax,PPCDroid/external-gst-openmax,felipec/gst-openmax,prajnashi/gst-openmax,mrchapp/gst-openmax-devel,freedesktop-unofficial-mirror/gstreamer__attic__gst-openmax,mrchapp/gst-openmax-devel,mrchapp/gst-openmax,prajnashi/gst-openmax,mrchapp/gst-openmax-devel,mrchapp/gst-openmax,PPCDroid/external-gst-openmax,felipec/gst-openmax,freedesktop-unofficial-mirror/gstreamer__attic__gst-openmax,matsu/gst-openmax,matsu/gst-openmax