This commit fixes a number of bugs:

1. There was no check that no read error/EOF occured during
ebml_read_uint, ebml_read_sint and ebml_read_float.
2. ebml_read_ascii and ebml_read_binary did sometimes not forward
error codes; instead they simply returned AVERROR(EIO).
3. In particular, AVERROR_EOF hasn't been used and no dedicated error
message for it existed. This has been changed.

In order to reduce code duplication, the new error code NEEDS_CHECKING
has been introduced which makes ebml_parse check the AVIOContext's
status for errors.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>