ext/pango/gsttextoverlay.c: Fix invalid memory access (we can't access a buffer after it's been pushed downstream wit...

Original commit message from CVS:
* ext/pango/gsttextoverlay.c: (gst_text_overlay_render_text),
(gst_text_overlay_video_chain):
Fix invalid memory access (we can't access a buffer after it's been
pushed downstream without taking a reference); fix memory leak (if
there's no text to render, bail out before allocating stuff).