If there is e.g. ESC [ in the input buffer, combine that to Alt-[.
Check the first character too; don't blindly assume it is ESC, as
it can be NUL as well. Note this means you can no longer activate
the main menu by pressing Ctrl-@ (or Ctrl-Space on some terminals).
Otherwise, the timeout could cause ELinks to resume reading from
the terminal device while another process is still using it.
This actually happened in a test.
On entry to some functions that could resume reading from the device,
assert that the terminal has not been blocked.
