Fix xsparse.c big heap allocation bugs

Fix xsparse.c big heap allocation bugs

* scripts/xsparse.c (expand_sparse): Read into auto buffer, not heap.
The heap code was wrong for two reasons: it called malloc just once
in the try-again loop, and even when it succeeded it could have
left so few bytes available in the heap that later stdio calls
could fail.  Reading into the auto buffer might be a bit slower
but speed is not an issue here and it’s better to be simple.