Fix pyraise (#675)

Author: tkf

src/exception.jl

@@ -180,9 +180,9 @@ function pyraise(e::PyError, ::Vector = [])
     ccall((@pysym :PyErr_Restore), Cvoid, (PyPtr, PyPtr, PyPtr),
           e.T, e.val, e.traceback)
     # refs were stolen
-    setfield!(e.T, :o, C_NULL)
-    setfield!(e.val, :o, C_NULL)
-    setfield!(e.traceback, :o, C_NULL)
+    setfield!(e.T, :o, PyPtr_NULL)
+    setfield!(e.val, :o, PyPtr_NULL)
+    setfield!(e.traceback, :o, PyPtr_NULL)
 end
 
 """