roken: fix 0acef7729f

Should have used -- instead of == in the prior commit.

Change-Id: I90f8886b754dda19970c6579ffa477634e8dc4a5

lib/roken/snprintf.c

@@ -276,7 +276,7 @@ append_string (struct snprintf_state *state,
 	len += pad(state, width, ' ');
 
     if (prec != -1) {
-	while (prec== && *arg) {
+	while (prec-- && *arg) {
 	    (*state->append_char) (state, *arg++);
 	    ++len;
 	}