json: Add fixup_free function for json_get_field

(cherry picked from commit 305dbe7197f87d8e14bb75f14553852c8e3c9b8c)

src/modules/json/json_mod.c

@@ -82,7 +82,7 @@ static int fixup_get_field(void** param, int param_no)
 static int fixup_get_field_free(void** param, int param_no)
 {
 	if (param_no == 1 || param_no == 2) {
-		LM_WARN("free function has not been defined for spve\n");
+		fixup_free_spve_null(param, 1);
 		return 0;
 	}