bytestream2_get_bytes_left returns an unsigned int; as a result,
it returns big positive numbers if an overread already happened,
making it unsuitable for scenarios where one wants to allow this
in a controlled way (because the buffer is actually padded so that
no segfaults can happen). So change it to return an ordinary int.

Also, bytestream2_get_bytes_left_p has been modified in the same way.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>