ptask: remove usage of nonexistent function is_error

see https://github.com/json-c/json-c/issues/304

pkgs/applications/misc/ptask/default.nix

···
 
   nativeBuildInputs = [ pkgconfig makeWrapper ];
 
-  patches = [ ./tw-version.patch ];
+  patches = [ ./tw-version.patch ./json_c_is_error.patch ];
 
   preFixup = ''
     wrapProgram "$out/bin/ptask" \

pkgs/applications/misc/ptask/json_c_is_error.patch

···
+diff --git a/src/tw.c b/src/tw.c
+index 602f7b3..ca601cd 100644
+--- a/src/tw.c
++++ b/src/tw.c
+@@ -163,7 +163,7 @@ static struct json_object *task_exec_json(const char *opts)
+ 
+ 	free(cmd);
+ 
+-	if (o && is_error(o))
++	if (!o)
+ 		return NULL;
+ 
+ 	return o;