Previously, it was possible to enter ‘ ’ (a space) at a number prompt,
and it would be stripped, passed to is_number() (which would erroneously
return TRUE), then passed to strtol() and the return value used
unconditionally.
Fix that by fixing is_number() to return FALSE for the empty string.
Signed-off-by: Philip Withnall <withnall@endlessm.com>