crypto/rand: better panic message for invalid Int argument.

Also document the panic to be consistent with math/rand.

Fixes #5187.

R=golang-dev, dave, bradfitz, r
CC=golang-dev
https://golang.org/cl/8303043

src/pkg/crypto/rand/util.go

@@ -100,8 +100,11 @@ func Prime(rand io.Reader, bits int) (p *big.Int, err error) {
 	}
 }
 
-// Int returns a uniform random value in [0, max).
+// Int returns a uniform random value in [0, max). It panics if max <= 0.
 func Int(rand io.Reader, max *big.Int) (n *big.Int, err error) {
+	if max.Sign() <= 0 {
+		panic("crypto/rand: argument to Int is <= 0")
+	}
 	k := (max.BitLen() + 7) / 8
 
 	// b is the number of bits in the most significant byte of max.