avformat/swfdec: Reorder allocations/initializations

avformat/swfdec: Reorder allocations/initializations

The earlier code would first attempt to allocate two buffers, then
attempt to allocate an AVIOContext, using one of the new buffers I/O
buffer, then check the allocations. On success, a z_stream that is used
in the AVIOContext's read_packet callback is initialized afterwards.

There are two problems with this: In case the allocation of the I/O
buffer fails avio_alloc_context() will be given a NULL read buffer
with a size > 0. This works right now, but it is fragile. The second
problem is that the z_stream used in the read_packet callback is not
functional when avio_alloc_context() is allocated (it might be that
avio_alloc_context() might already fill the buffer in the future). This
commit fixes both of these problems by reordering the operations.

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