common: readline: Fix always true test

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: dcc18ce0dbaf ("cli: Implement delete-word in cread_line()")
Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>
diff --git a/common/cli_readline.c b/common/cli_readline.c
index 4e6797a..0cb43e6 100644
--- a/common/cli_readline.c
+++ b/common/cli_readline.c
@@ -332,8 +332,8 @@
 		if (cls->num) {
 			uint base, wlen;
 
-			for (base = cls->num - 1;
-			     base >= 0 && buf[base] == ' ';)
+			for (base = cls->num;
+			     base > 0 && buf[base - 1] == ' ';)
 				base--;
 			for (; base > 0 && buf[base - 1] != ' ';)
 				base--;