avformat/matroskadec: Reuse positions

avformat/matroskadec: Reuse positions

Up until now, avio_tell was used multiple times in ebml_parse and its
subroutines, although the result of these calls can usually be simply
derived from the result of earlier calls to avio_tell. This has been
changed. Unnecessary calls to avio_tell in ebml_parse are avoided now.

Furthermore, there has been a slight change in the output of some error
messages relating to elements exceeding their containing master element:
The reported position of the element now points to the first byte of the
element ID and no longer to the first byte of the element's payload.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>