return a span from TORRENT_ALLOCA
Unfortunately this requires moving the variable declaration inside the macro.
Due to alloca’s unique properties pretty much the only safe way to call it is
in a simple assign-to-pointer expression. Therefor we need to use a temporary
pointer to store the value returned from alloca before we can call span’s ctor.
This also causes double evaluation of the size parameter which is unfortunate,
but no current callers of TORRENT_ALLOCA have a problem with this. Passing
expressions with side effects to macros is bad mojo anyways.
Steven Siloti