Use BN_CTX_end when exiting early from BN_mod_exp_mont_word because

BN_mod_exp_atalla could be used.

crypto/bn/bn_exp.c

@@ -692,8 +692,11 @@ int BN_mod_exp_mont_word(BIGNUM *rr, BN_ULONG a, const BIGNUM *p,
 		{
 		BN_set_word(t, a);
 		if (BN_mod_exp_atalla(rr, t, p, m))
+			{
+			BN_CTX_end(ctx);
 			return 1;
 			}
+		}
 /* If it fails, try the other methods */
 #endif