add -p: avoid use of undefined $key when ReadKey -> EOF

add -p: avoid use of undefined $key when ReadKey -> EOF

b5cc003253 (add -i: ignore terminal escape sequences, 2011-05-17)
add an additional check to the original code to better handle keys
for escape sequences, but failed to account for the possibility
the first ReadKey call returned undef (ex: stdin closes) and that
was being handled fine by the original code in ca6ac7f135 (add -p:
prompt for single characters, 2009-02-05)

Add a test for undefined and encapsulate the loop and the original
print that relied on it within it.

After this, the following command (in a suitable repository state)
wouldn't print any error:

  $ git -c interactive.singleKey add -p </dev/null

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>