small bug in _sqrt function

[HadarIngonyama]

The Euler criterion is for n!=0.
I added a fix to deal with the case n==0.

def _sqrt(n, p, sign=0):
    """ Generic Tonelli–Shanks algorithm """
    if n == 0:
        return 0

    #check Euler criterion
    if pow(n,(p-1)//2,p) != 1:
        return None

    #compute square root
    p_1 = p-1
    s = 0
    q = p_1
    while q & 1 == 0:
        q = q>>1
        s  = s+1
    if s == 1:
        r = pow(n,(p+1)//4,p)
    else:
        z = 2
        while pow(z,(p-1)//2,p) == 1:
            z = z+1
        c = pow(z,q,p)
        r = pow(n,(q+1)//2,p)
        t = pow(n,q,p)
        m = s
        while True:
            if t == 1:
                break
            else:
                for i in range(1,m):
                    if pow(t,pow(2,i),p) == 1:
                        break
                b = pow(c,pow(2,m-i-1),p)
                r = (r*b)   %p
                t = (t*b*b) %p
                c = (b*b)   %p
                m = i
    if sign:
        sign = 1
    if r&1 != sign:
        r = p-r
    return r