gdb/tui: Prevent exceptions from trying to cross readline

gdb/tui: Prevent exceptions from trying to cross readline

This is triggered by simply scrolling off the end of the dissasembly
window.  This commit doesn't fix the actual exception that is being
thrown, which will still need to be fixed, but makes sure that we
don't ever throw an exception out to readline.

gdb/ChangeLog:
yyyy-mm-dd  Pedro Alves  <palves@redhat.com>

        PR tui/9765
        * tui/tui-io.c (tui_getc): Rename to ...
        (tui_getc_1): ... this.
        (tui_get): New, reimplent as try/catch wrapper around tui_getc_1.

Change-Id: I2e32a401ab34404b2132ec82a3e1c17b9b723e41