gst/subparse/gstsubparse.c: Add missing closing tags for markup and fix broken markup, otherwise pango won't render a...

Original commit message from CVS:
* gst/subparse/gstsubparse.c: (subrip_fix_up_markup),
(parse_subrip), (handle_buffer):
Add missing closing tags for markup and fix broken markup,
otherwise pango won't render anything (fixes #357531). Also,
make sure the text we send out is always NUL-terminated
(better safe than sorry etc.).
* tests/check/elements/subparse.c: (test_srt_do_test),
(test_srt):
Some more tests for .srt incl. tests for the above stuff.