Add test case for outputJSON

Fixes #856

changes.rst

@@ -8,14 +8,15 @@ https://github.com/MiniZinc/libminizinc/issues.
 
 Changes:
 ^^^^^^^^
--  Add support for `elseif` in `if then elseif endif` expressions without an
+-  Add support for ``elseif`` in ``if then elseif endif`` expressions without an
    `else` branch.
 
 Bug fixes:
 ^^^^^^^^^^
 -  Fix boolean context when flattening tuple or record literals containing
    booleans.
 -  Fix memory leak in overflow handler.
+-  Fix crash when calling ``outputJSON`` (:bugref:`856`).
 
 .. _v2.8.7:
 

tests/spec/unit/regression/github_856.mzn

@@ -0,0 +1,10 @@
+/***
+!Test
+solvers: [gecode]
+expected: !Result
+  solution: !Solution
+    _output_item: "{\n  \"x\" : 1\n}\n"
+***/
+
+var 1..1: x;
+output outputJSON();