Check that key was decoded from UTF-8

Author: oschwald

extension/maxminddb.c

@@ -526,6 +526,11 @@ static PyObject *from_map(MMDB_entry_data_list_s **entry_data_list) {
         PyObject *key = PyUnicode_FromStringAndSize(
             (char *)(*entry_data_list)->entry_data.utf8_string,
             (*entry_data_list)->entry_data.data_size);
+        if (!key) {
+            // PyUnicode_FromStringAndSize will set an appropriate exception
+            // in this case.
+            return NULL;
+        }
 
         *entry_data_list = (*entry_data_list)->next;