Fix multi-threading issue

yara_callback was invoking Python's code without acquiring the global interpreter lock.

yara-python.c

@@ -429,11 +429,12 @@ int yara_callback(
   if (message == CALLBACK_MSG_RULE_NOT_MATCHING && callback == NULL)
     return CALLBACK_CONTINUE;
 
+  if (message == CALLBACK_MSG_IMPORT_MODULE && modules_data == NULL)
+    return CALLBACK_CONTINUE
+
   if (message == CALLBACK_MSG_IMPORT_MODULE)
   {
-    if (modules_data == NULL)
-      return CALLBACK_CONTINUE;
-
+    gil_state = PyGILState_Ensure();
     module_import = (YR_MODULE_IMPORT*) message_data;
 
     module_data = PyDict_GetItemString(
@@ -461,6 +462,7 @@ int yara_callback(
       module_import->module_data_size = data_size;
     }
 
+    PyGILState_Release(gil_state);
     return CALLBACK_CONTINUE;
   }