fix metal uaf (#964)

test/external/external_metal_uaf.py

@@ -0,0 +1,17 @@
+import weakref
+import numpy as np
+from tinygrad.tensor import Tensor, Device
+Device.DEFAULT = "METAL"
+
+if __name__ == "__main__":
+  t = Tensor.zeros(3).realize()
+  wt = weakref.ref(t.lazydata.realized)
+  n = t.numpy()
+  t += 1
+  n2 = t.numpy()
+  print(wt)
+  del t
+  print(wt)
+  print(n, n.base, n.base.base)
+  print(n2, n2.base, n2.base.base)
+  assert wt() is not None