[flang] Prevent segfault in DynamicType::IsTkCompatibleWith

Make sure `derived_` is not null before dereferencing it.

This might only happen when there is a previous error. I saw it
compiling a test program where a USEd module was not found.

Original-commit: flang-compiler/f18@2db516556b1509a8519ce91013f49431b6d43d22
Reviewed-on: https://github.com/flang-compiler/f18/pull/550