Set the global bridge instance to null on deinit

Hosts like Qtractor will deinitialize the module only to then
immediately initialize it again, so without this we would get an
assertion failure in `InitModule()`.

src/plugin/vst3-plugin.cpp

@@ -54,6 +54,8 @@ bool DeinitModule() {
     assert(bridge != nullptr);
 
     delete bridge;
+    bridge = nullptr;
+
     return true;
 }