Take read lock before checking if closed

This prevents a race. We also check that the database is closed in two
other places, but neither of those actually read from the database.

Author: oschwald

extension/maxminddb.c

@@ -705,13 +705,15 @@ static bool is_ipv6(char ip[16]) {
 
 static PyObject *ReaderIter_next(PyObject *self) {
     ReaderIter_obj *ri = (ReaderIter_obj *)self;
-    if (ri->reader->closed == Py_True) {
-        PyErr_SetString(PyExc_ValueError,
-                        "Attempt to iterate over a closed MaxMind DB.");
+
+    if (reader_acquire_read_lock(ri->reader) != 0) {
         return NULL;
     }
 
-    if (reader_acquire_read_lock(ri->reader) != 0) {
+    if (ri->reader->closed == Py_True) {
+        reader_release_read_lock(ri->reader);
+        PyErr_SetString(PyExc_ValueError,
+                        "Attempt to iterate over a closed MaxMind DB.");
         return NULL;
     }