crypto: lib/mpi - avoid null pointer deref in mpi_cmp_ui()

During NVMeTCP Authentication a controller can trigger a kernel
oops by specifying the 8192 bit Diffie Hellman group and passing
a correctly sized, but zeroed Diffie Hellamn value.
mpi_cmp_ui() was detecting this if the second parameter was 0,
but 1 is passed from dh_is_pubkey_valid(). This causes the null
pointer u->d to be dereferenced towards the end of mpi_cmp_ui()

Signed-off-by: Mark O'Donovan <shiftee@posteo.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

lib/crypto/mpi/mpi-cmp.c

@@ -25,8 +25,12 @@ int mpi_cmp_ui(MPI u, unsigned long v)
 	mpi_limb_t limb = v;
 
 	mpi_normalize(u);
-	if (!u->nlimbs && !limb)
-		return 0;
+	if (u->nlimbs == 0) {
+		if (v == 0)
+			return 0;
+		else
+			return -1;
+	}
 	if (u->sign)
 		return -1;
 	if (u->nlimbs > 1)