pyrdb: casting null should return null

This fixes a crash when a type in a slotted hierarchy has no slots, like
_socket.socket.  Should generally make things better

Author: peadar

libpstack/pyrdb.h

@@ -330,8 +330,10 @@ class Target {
 
 
 template<typename To> Remote<To *> Target::cast(const PyType<To> &to, Remote<PyObject *> from) const {
-    if (pyType(from) == to.typeObject)
-        return { reinterpret_cast<To *>(from.remote) };
+    if (from) {
+        if (pyType(from) == to.typeObject)
+            return { reinterpret_cast<To *>(from.remote) };
+    }
     return {0};
 }