avoid invalid free if out of memory

The surrounding code suggests this should only be freed if it was allocated
locally.

src/_imaging.c

@@ -778,6 +778,7 @@ _prepare_lut_table(PyObject* table, Py_ssize_t table_size)
     /* malloc check ok, max is 2 * 4 * 65**3 = 2197000 */
     prepared = (INT16*) malloc(sizeof(INT16) * table_size);
     if ( ! prepared) {
+        if (free_table_data)
             free(table_data);
         return (INT16*) PyErr_NoMemory();
     }