Fix a memory leak in JV and add more tests

c/testdata

@@ -215,6 +215,14 @@ def id(x):x; 2000 as $x | def f(x):1 as $x | id([$x, x, x]); def g(x): 100 as $x
 {"bar":42}
 {"foo":42, "bar":42}
 
+.foo |= .+1
+{"foo": 42}
+{"foo": 43}
+
+.[0].a |= {"old":., "new":(.+1)}
+[{"a":1,"b":2}]
+[{"a":{"old":1, "new":2},"b":2}]
+
 def inc(x): x |= .+1; inc(.[].a)
 [{"a":1,"b":2},{"a":2,"b":4},{"a":7,"b":8}]
 [{"a":2,"b":2},{"a":3,"b":4},{"a":8,"b":8}]