[rng] Record validity within DRBG state

Treat an empty (zeroed) DRBG as invalid.  This ensures that a DRBG
that has not yet been instantiated (or that has been uninstantiated)
will refuse to attempt to generate random bits.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/drbg.h

@@ -39,6 +39,8 @@ struct drbg_state {
 	struct hmac_drbg_state internal;
 	/** Reseed required flag */
 	int reseed_required;
+	/** State is valid */
+	int valid;
 };
 
 /**