Fix memleak caused by the fact that url_open_buf() allocates a context

Fix memleak caused by the fact that url_open_buf() allocates a context
when calling, but url_close_buf() doesn't free it. The better solution
is to not allocate it at all, init it with init_put_byte() and then
not have to close it at all. In the case where we do need to hold it
around for longer than within the function context, we allocate it with
av_alloc_put_byte() and free it with av_free() instead. Discussed in ML
thread "[PATCH] fix small memleak in rdt.c".

Originally committed as revision 15850 to svn://svn.ffmpeg.org/ffmpeg/trunk