[gdb/tui] Clean up asserts in tui_source_window_base::refresh_window

[gdb/tui] Clean up asserts in tui_source_window_base::refresh_window

Commit 1c525b0e037 ("[gdb/tui] Fix assert in
tui_source_window_base::refresh_window") added an early return in
tui_source_window_base::refresh_window.

Assert after the early return that "m_pad != nullptr", and clean up the
following asserts that allow for m_pad == nullptr.

Tested on x86_64-linux.

Reported-By: Andrew Burgess <aburgess@redhat.com>
Co-Authored-By: Andrew Burgess <aburgess@redhat.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>