streamsynchronizer: don't send gap events with huge bogus durations when advancing...

streamsynchronizer: don't send gap events with huge bogus durations when advancing EOS streams

When the input buffers for a stream don't have a duration set,
timestamp_end might still be GST_CLOCK_TIME_NONE. When advancing
EOSed streams via GAP events (with other streams not yet EOS), we
would then use the invalid timestamp_end to calculate the duration
of the gap. This in turn would make baseaudiosink abort, because it
would try to allocate memory for a trizillion samples.

So if buffers don't have a duration set, assume a duration of
one second for stream catch-up purposes, just so we can still
continue to catch up in those cases. And make sure that
timestamp_end is valid before doing calculations with it.

http://bugzilla.gnome.org/show_bug.cgi?id=678530