ff_flac_parse_picture() parses a buffer containing a flac metadata
picture block by wrapping it in an AVIOContext and using the AVIOContext
API. Consequently, when not enough data could be read AVERROR(EIO) was
returned although reading didn't really fail: A block that contains a
subfield whose size field indicates that it is so big as to extend
beyond the buffer is just invalid.

This commit changes this by using the bytestream2 API instead;
furthermore, the checks for whether there is enough data left are
performed before allocating a buffer for said data.

Finally, if the length of the picture description is bigger than
INT_MAX, it will now raise an error.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>