avoid potential infinite loop in final reduction round of BN_GF2m_mod_arr()

Submitted by: Huang Ying
Reviewed by: Douglas Stebila

CHANGES

@@ -702,12 +702,16 @@
 
  Changes between 0.9.8h and 0.9.8i  [xx XXX xxxx]
 
+  *) Fix BN_GF2m_mod_arr() top-bit cleanup code.
+     [Huang Ying]
+
   *) Expand ENGINE to support engine supplied SSL client certificate functions.
 
      This work was sponsored by Logica.
      [Steve Henson]
 
-  *) Add CryptoAPI ENGINE to support use of RSA and DSA keys held in Windows         keystores. Support for SSL/TLS client authentication too.
+  *) Add CryptoAPI ENGINE to support use of RSA and DSA keys held in Windows
+     keystores. Support for SSL/TLS client authentication too.
      Not compiled unless enable-capieng specified to Configure.
 
      This work was sponsored by Logica.

crypto/bn/bn_gf2m.c

@@ -322,7 +322,11 @@ int BN_GF2m_mod_arr(BIGNUM *r, const BIGNUM *a, const int p[])
 		if (zz == 0) break;
 		d1 = BN_BITS2 - d0;
 		
-		if (d0) z[dN] = (z[dN] << d1) >> d1; /* clear up the top d1 bits */
+		/* clear up the top d1 bits */
+		if (d0)
+			z[dN] = (z[dN] << d1) >> d1;
+		else
+			z[dN] = 0;
 		z[0] ^= zz; /* reduction t^0 component */
 
 		for (k = 1; p[k] != 0; k++)