Warn on NewArray with potentially negative expr

[problame]

class Main {
    public static void main(String[] args) {
        int x = System.in.read();
        int[] foo = new int[x];

        if y = System.in.read();
        if (y <= 0) {
           return;
        }
        int[] bar = new int[y];
   }
}

x could be negative. Warn for that line.

y cannot be negative after the if stmt, so don't warn there.