rsa2_pubkey_bits: Cope correctly with a NULL return from rsa2_newkey()

Dereferencing it is not correct.
Bug found with the help of afl-fuzz.

sshrsa.c

@@ -773,6 +773,8 @@ static int rsa2_pubkey_bits(const struct ssh_signkey *self,
     int ret;
 
     rsa = rsa2_newkey(self, (const char *) blob, len);
+    if (!rsa)
+	return -1;
     ret = bignum_bitcount(rsa->modulus);
     rsa2_freekey(rsa);