media_editor_private: Clear event source not fired before overwriting it
It is similar with
a9e2382ff2be2d2ddf13bdd8bbb261a208c03f82 commit
of capi-media-webrtc.
[Version] 0.1.7
[Issue Type] Improvement
Change-Id: I61d5836af104e785a9f9e2318f5637ce35f34a92
Signed-off-by: Sangchul Lee <sc11.lee@samsung.com>