drbg: strength is initialized before check?

[Kris Kwiatkowski]

Seems like a bug:

	// Minimum entropy input (SP800-90A, 10.2.1)
	if len(entropy) < int(c.strength/8) {
		return false
	}

	// Security strength for AES-256 as per SP800-57, 5.6.1
	c.strength = 256