Fix parsing of function declarations that return pointers.

Check that <op>= only evaluates the left-hand-side once.
diff --git a/libacc/tests/test.py b/libacc/tests/test.py
index 4be1f4b..7bcbdd4 100644
--- a/libacc/tests/test.py
+++ b/libacc/tests/test.py
@@ -356,6 +356,13 @@
 17&= 1  1
 17^= 1  16
 16|= 1  17
+*f() = *f() + 10;
+f()
+f()
+a = 10
+*f() += 10;
+f()
+a = 10
 """)
 
     def testcomma(self):