vobsub_read_header() uses an AVBPrint to write a string and up until
now, it collected the string stored in the AVBPrint via
av_bprint_finalize(), which might involve an allocation and copy of the
string. But this is unnecessary, as the lifetime of the returned string
does not exceed the lifetime of the AVBPrint. So use the string in the
AVBPrint directly.

This also makes it possible to easily fix a memleak: In certain error
situations, the string stored in the AVBPrint would not be freed (if it
was dynamically allocated). This has been fixed, too.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>