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When the user changes her mind after having pressed Esc, she cannot
unpress Esc, so do that for her by ignoring the escape code when
the subsequent keycode is outside of the normal printable range.

This restores the behavior from before nano-2.3.5 -- except that
Ctrl+Alt+key continues to report an unbound key.

This fixes https://savannah.gnu.org/bugs/?48459.