Properly clear the last entered character

When enter is pressed, passwd[len] will be set to '\0'. Pressing
backspace is supposed to remove the last entered character.

But currently, the clearing has an off-by-one, as in setting
passwd[len] to '\0' just like enter would do.

You can also verify it by imagining len=1 and that it's impossible to
clear passwd[0] by pressing backspace with the current code.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>

slock.c

@@ -177,7 +177,7 @@ readpw(Display *dpy, struct xrandr *rr, struct lock **locks, int nscreens,
 				break;
 			case XK_BackSpace:
 				if (len)
-					passwd[len--] = '\0';
+					passwd[--len] = '\0';
 				break;
 			default:
 				if (num && !iscntrl((int)buf[0]) &&