read_stream_list(): Spill huge data ranges array onto heap

read_stream_list():  Spill huge data ranges array onto heap

There can be thousands of streams packed into a single resource, so it's
a good idea to check if the data_ranges array is really small enough to
be allocated on the stack or not, and if not, allocate it on the heap
instead.

For readability and to create a temporary frame for alloca(), place the
relevant code in a new function read_packed_streams().