Assert that we don't try to have an ArenaBlock depend on itself.

It introduces a cycle in the `next` pointers in the ArenaBlockRef's,
which results in infinite recursion as we follow them in `getTotalSize`.

This doesn't fix the underlying problem, but gives a better failure
than a stack overflow.

flow/Arena.cpp

@@ -297,6 +297,7 @@ void* ArenaBlock::make4kAlignedBuffer(uint32_t size) {
 }
 
 void ArenaBlock::dependOn(Reference<ArenaBlock>& self, ArenaBlock* other) {
+	ASSERT(self->getData() != other->getData());
 	other->addref();
 	if (!self || self->isTiny() || self->unused() < sizeof(ArenaBlockRef))
 		create(SMALL, self)->makeReference(other);