bn/bn_gf2m.c: make new BN_GF2m_mod_inv work with BN_DEBUG_RAND.

crypto/bn/bn_gf2m.c

@@ -631,8 +631,11 @@ int BN_GF2m_mod_inv(BIGNUM *r, const BIGNUM *a, const BIGNUM *p, BN_CTX *ctx)
 			}
 		if (ubits==vbits)
 			{
-			bn_correct_top(u);
-			ubits = BN_num_bits(u);
+			BN_ULONG u;
+			int utop = (ubits-1)/BN_BITS2;
+
+			while ((u=udp[utop])==0 && utop) utop--;
+			ubits = utop*BN_BITS2 + BN_num_bits_word(u);
 			}
 		}
 	bn_correct_top(b);