qtdemux: fragmented seek fails and goes unnoticed

Submitted by Thiago Sousa Santos @thiagossantos

Link to original bug (#766084)

Description

When seeking on a fragmented file without proper seeking information, qtdemux will return TRUE to the seek event but when it actually tries to seek from the streaming thread it fails.

It ignored this failure, pushes a new segment event with the seek requested parameters and then goes pushing data from the same position where it was.
gst-integration-testsuites/medias/defaults/mp4/fragmented_nonseekable_sink.mp4
file from our validate tests.