Prevent handshake with unseeded PRNG

Fix security issue where under certain conditions a client can complete a
handshake with an unseeded PRNG. The conditions are:
- Client is on a platform where the PRNG has not been seeded, and the
user has not seeded manually
- A protocol specific client method version has been used (i.e. not
SSL_client_methodv23)
- A ciphersuite is used that does not require additional random data
from the PRNG beyond the initial ClientHello client random
(e.g. PSK-RC4-SHA)

If the handshake succeeds then the client random that has been used will
have been generated from a PRNG with insufficient entropy and therefore
the output may be predictable.

For example using the following command with an unseeded openssl will
succeed on an unpatched platform:

openssl s_client -psk 1a2b3c4d -tls1_2 -cipher PSK-RC4-SHA

CVE-2015-0285

Reviewed-by: Richard Levitte <levitte@openssl.org>
(cherry picked from commit e1b568dd2462f7cacf98f3d117936c34e2849a6b)

ssl/s3_clnt.c

@@ -717,8 +717,9 @@ int ssl3_client_hello(SSL *s)
         } else
             i = 1;
 
-        if (i)
-            ssl_fill_hello_random(s, 0, p, sizeof(s->s3->client_random));
+        if (i && ssl_fill_hello_random(s, 0, p,
+                                       sizeof(s->s3->client_random)) <= 0)
+            goto err;
 
         /* Do the message type and length last */
         d = p = ssl_handshake_start(s);