cli: Correct several bugs in cli_getch()

cli: Correct several bugs in cli_getch()

This function does not behave as expected when unknown escape sequences
are sent to it:

- it fails to store (and thus echo) the last character of the invalid
  sequence
- it fails to set esc_len to 0 when it finishes emitting the invalid
  sequence, meaning that the following character will appear to be part
  of a new escape sequence
- it processes the first character of the rejected sequence as a valid
  character, just starting the sequence all over again

The last two bugs conspire to produce an "impossible condition #876"
message which is the main symptom of this behaviour.

Fix these bugs and add a test to verify the behaviour.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>