VT: Remove assertion in send() function.

[Problem] Media server got crashed if wifi connection is disconnected
while ARTPWriter is sending rtp packets.

[Cause] There is an assertion condition in send().

[Solution] Remove assertion in send().

Merged-in: I24d9d673afed529e4e571f6fefc3b2ae0c7fef1a
Change-Id: I24d9d673afed529e4e571f6fefc3b2ae0c7fef1a
Signed-off-by: Byeongjo Park <bjo.park@samsung.com>