patch 8.0.0216: decoding js style json may fail

Problem:    When decoding JSON with a JS style object the JSON test may use a
            NULL pointer. (Coverity)
Solution:   Check for a NULL pointer.
diff --git a/src/json_test.c b/src/json_test.c
index 74463f3..c7779b2 100644
--- a/src/json_test.c
+++ b/src/json_test.c
@@ -107,6 +107,12 @@
     reader.js_buf = (char_u *)"  {   ";
     assert(json_find_end(&reader, 0) == MAYBE);
 
+    /* JS object with white space */
+    reader.js_buf = (char_u *)"  {  a  :  123  }  ";
+    assert(json_find_end(&reader, JSON_JS) == OK);
+    reader.js_buf = (char_u *)"  {  a  :   ";
+    assert(json_find_end(&reader, JSON_JS) == MAYBE);
+
     /* array without white space */
     reader.js_buf = (char_u *)"[\"a\",123]";
     assert(json_find_end(&reader, 0) == OK);