[gdb/tui] Fix displaying main after resizing
A TUI src window is displaying either:
- the source for the current frame,
- the source for main, or
- the string "[ No Source Available ]".
Since commit
03893ce67b5 ("[gdb/tui] Fix resizing of terminal to 1 or 2 lines")
we're able to resize the TUI to 1 line without crashing.
I noticed that if TUI is displaying main, and we resize to 1 line (destroying
the src window) and then back to a larger terminal (reconstructing the src
window), the TUI displays "[ No Source Available ]" instead of main.
Fix this by moving the responsibility for showing main from tui_enable to
tui_source_window_base::rerender.
Tested on x86_64-linux.
Approved-By: Tom Tromey <tom@tromey.com>