avcodec/movtextdec: Skip empty styles

avcodec/movtextdec: Skip empty styles

They would either lead to unnecessary ASS tags being emitted (namely
tags that are reset immediately thereafter) or would lead to problems
when parsing: e.g. if a zero-length style immediately follows another
style, the current code will end the preceding style and set the
zero-length style as the next potentially active style, but it is only
tested for activation when the next character is parsed at which point
the current offset is already greater than both the starting as well
as the end offset of the empty style. It will therefore neither be
opened nor closed and all subsequent styles will be ignored.

Reviewed-by: Philip Langdale <philipl@overt.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>