The variable base is unsigned so >= 0 is always true. Fix this test
so that it is actually useful. The fix prevents the code from causing
a segfault in the case where Ctrl-w is pressed on a line consisting
only of spaces.
Fixes: dcc18ce0db ("cli: Implement delete-word in cread_line()")
Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>