output:fix seg fault in transmitter when weston terminates

output:fix seg fault in transmitter when weston terminates

Bug-AGL: SPEC-2939

If we stop weston with kill command weston doesn't terminate
properly and receives segmentation fault beacause of double free
output->link which happened because of wrongly calling
weston_output_destroy inside transmitter_destroy function.
Also initialize transmitter_output_disable to base disable pointer.

Change-Id: I2adda6eba7d260398d3e871fc21bf8f04d59cf0b
Signed-off-by: Veeresh Kadasani <external.vkadasani@jp.adit-jv.com>
(cherry picked from commit 8409a6d28957c6edd9caae8143ee54877e0e1256)