[3.11] gh-99240: Fix double-free bug in Argument Clinic str_converter generated code (GH-99241) (#100352)

(cherry picked from commit 8dbe08eb7c)

Fix double-free bug mentioned at GH-99240, by moving memory clean up out of "exit" label.

Modules/_testclinic.c

@@ -551,6 +551,64 @@ error:
 }
 
 
+/*[clinic input]
+str_converter_encoding
+
+    a: str(encoding="idna")
+    b: str(encoding="idna", accept={bytes, bytearray, str})
+    c: str(encoding="idna", accept={bytes, bytearray, str}, zeroes=True)
+    /
+
+[clinic start generated code]*/
+
+static PyObject *
+str_converter_encoding_impl(PyObject *module, char *a, char *b, char *c,
+                            Py_ssize_t c_length)
+/*[clinic end generated code: output=af68766049248a1c input=0c5cf5159d0e870d]*/
+{
+    assert(!PyErr_Occurred());
+    PyObject *out[3] = {NULL,};
+    int i = 0;
+    PyObject *arg;
+
+    arg = PyUnicode_FromString(a);
+    assert(arg || PyErr_Occurred());
+    if (!arg) {
+        goto error;
+    }
+    out[i++] = arg;
+
+    arg = PyUnicode_FromString(b);
+    assert(arg || PyErr_Occurred());
+    if (!arg) {
+        goto error;
+    }
+    out[i++] = arg;
+
+    arg = PyUnicode_FromStringAndSize(c, c_length);
+    assert(arg || PyErr_Occurred());
+    if (!arg) {
+        goto error;
+    }
+    out[i++] = arg;
+
+    PyObject *tuple = PyTuple_New(3);
+    if (!tuple) {
+        goto error;
+    }
+    for (int j = 0; j < 3; j++) {
+        PyTuple_SET_ITEM(tuple, j, out[j]);
+    }
+    return tuple;
+
+error:
+    for (int j = 0; j < i; j++) {
+        Py_DECREF(out[j]);
+    }
+    return NULL;
+}
+
+
 static PyObject *
 bytes_from_buffer(Py_buffer *buf)
 {
@@ -927,6 +985,25 @@ gh_99233_refcount_impl(PyObject *module, PyObject *args)
 }
 
 
+/*[clinic input]
+gh_99240_double_free
+
+    a: str(encoding="idna")
+    b: str(encoding="idna")
+    /
+
+Proof-of-concept of GH-99240 double-free bug.
+
+[clinic start generated code]*/
+
+static PyObject *
+gh_99240_double_free_impl(PyObject *module, char *a, char *b)
+/*[clinic end generated code: output=586dc714992fe2ed input=23db44aa91870fc7]*/
+{
+    Py_RETURN_NONE;
+}
+
+
 static PyMethodDef tester_methods[] = {
     TEST_EMPTY_FUNCTION_METHODDEF
     OBJECTS_CONVERTER_METHODDEF
@@ -951,6 +1028,7 @@ static PyMethodDef tester_methods[] = {
     DOUBLE_CONVERTER_METHODDEF
     PY_COMPLEX_CONVERTER_METHODDEF
     STR_CONVERTER_METHODDEF
+    STR_CONVERTER_ENCODING_METHODDEF
     PY_BUFFER_CONVERTER_METHODDEF
     KEYWORDS_METHODDEF
     KEYWORDS_KWONLY_METHODDEF
@@ -970,6 +1048,7 @@ static PyMethodDef tester_methods[] = {
     KEYWORD_ONLY_PARAMETER_METHODDEF
     VARARG_AND_POSONLY_METHODDEF
     GH_99233_REFCOUNT_METHODDEF
+    GH_99240_DOUBLE_FREE_METHODDEF
     {NULL, NULL}
 };