Correct the logic of ready_for_new_data.

This should be set right after decoder really start to decode frame
instead setting at the end.

Even decoder does not have a displayable frame to show and return NULL
to application, this should be set too.

Change-Id: If0313a834bc64e3b0f05a84f4459d444d9eab0d8
Merge "Correct the logic of ready_for_new_data."