fix: release the allocator when an updated value escapes to a query

The allocator of _modify tracks the values it created so that a later
update can modify them in place instead of copying them. When a target
path is a proper prefix of an earlier one, getpath hands such a value to
the update function, which may store it in more than one place; updating
through one of those places then changes all of them.

    $ gojq -cn '{"a":{"b":1}} | (.a.b, .a, .a.p.z) |= {p: ., q: .}'

.a.p and .a.q hold separate values in jq, so only .a.p gains the "z" key,
while gojq adds it to both. Release the allocator whenever a value it
created leaves the update, so that the values are copied on update from
then on. The same applies to an object used as a path element, which
stays reachable through the path after the update.

cli/test.yaml

@@ -5502,6 +5502,14 @@
   expected: |
     [1,3,6,5,5]
 
+- name: update-assignment operator against object with overlapping paths
+  args:
+    - -c
+    - '(.a.b,.a,.a.p.z) |= {p: ., q: .}'
+  input: '{"a":{"b":1}}'
+  expected: |
+    {"a":{"p":{"b":{"p":1,"q":1},"z":{"p":null,"q":null}},"q":{"b":{"p":1,"q":1}}}}
+
 - name: update-assignment operator with optional operator
   args:
     - -c