Most of the data in the temporary storage ends up being
returned to the user as AVPacket.data, so it makes sense
to avoid using the AVBPrint for temporary storage altogether
(in particular in light of the fact that the blocks read here
are too big for the small-string optimization anyway) and
read the data directly into AVPacket.data. This also avoids
another memcpy() from a stack buffer to the AVBPrint in ts_image()
(that could always have been avoided with av_bprint_get_buffer()).

These changes also allow to use av_append_packet(), which
greatly simplifies the code; furthermore, one can avoid cleanup
code on error as the packet is already unreferenced generically
on error.

There are two user-visible changes from this patch:
1. Truncated packets are now marked as corrupt.
2. AVPacket.pos is set (it corresponds to the discarded header
line, 80 bytes before the position corresponding to the
actual packet data).

Furthermore, this patch also removes code that triggered
a -Wtautological-constant-out-of-range-compare warning
from Clang (namely a comparison of an unsigned and INT64_MAX
in an assert).
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>