Cherry-pick security-relevant fixes from upstream imath library.

This covers alterations to buffer sizing and zeroing made between imath
1.3 and imath 1.20.  Valgrind Memcheck identified the buffer overruns
and reliance on uninitialized data; their exploit potential is unknown.
Builds specifying --with-openssl are unaffected, because they use the
OpenSSL BIGNUM facility instead of imath.  Back-patch to 9.0 (all
supported versions).

Security: CVE-2015-0243

contrib/pgcrypto/imath.c

@@ -818,7 +818,8 @@ mp_int_mul(mp_int a, mp_int b, mp_int c)
 	 */
 	ua = MP_USED(a);
 	ub = MP_USED(b);
-	osize = ua + ub;
+	osize = MAX(ua, ub);
+	osize = 4 * ((osize + 1) / 2);
 
 	if (c == a || c == b)
 	{
@@ -907,7 +908,7 @@ mp_int_sqr(mp_int a, mp_int c)
 	CHECK(a != NULL && c != NULL);
 
 	/* Get a temporary buffer big enough to hold the result */
-	osize = (mp_size) 2 *MP_USED(a);
+	osize = (mp_size) 4 *((MP_USED(a) + 1) / 2);
 
 	if (a == c)
 	{
@@ -2605,8 +2606,8 @@ s_kmul(mp_digit *da, mp_digit *db, mp_digit *dc,
 		 * Now we'll get t1 = a0b0 and t2 = a1b1, and subtract them out so
 		 * that we're left with only the pieces we want:  t3 = a1b0 + a0b1
 		 */
-		ZERO(t1, bot_size + 1);
-		ZERO(t2, bot_size + 1);
+		ZERO(t1, buf_size);
+		ZERO(t2, buf_size);
 		(void) s_kmul(da, db, t1, bot_size, bot_size);	/* t1 = a0 * b0 */
 		(void) s_kmul(a_top, b_top, t2, at_size, bt_size);		/* t2 = a1 * b1 */
 
@@ -2616,11 +2617,13 @@ s_kmul(mp_digit *da, mp_digit *db, mp_digit *dc,
 
 		/* Assemble the output value */
 		COPY(t1, dc, buf_size);
-		(void) s_uadd(t3, dc + bot_size, dc + bot_size,
-					  buf_size + 1, buf_size + 1);
+		carry = s_uadd(t3, dc + bot_size, dc + bot_size,
+					   buf_size + 1, buf_size);
+		assert(carry == 0);
 
-		(void) s_uadd(t2, dc + 2 * bot_size, dc + 2 * bot_size,
+		carry = s_uadd(t2, dc + 2 * bot_size, dc + 2 * bot_size,
 					   buf_size, buf_size);
+		assert(carry == 0);
 
 		s_free(t1);				/* note t2 and t3 are just internal pointers
 								 * to t1 */
@@ -3307,7 +3310,10 @@ s_embar(mp_int a, mp_int b, mp_int m, mp_int mu, mp_int c)
 	dbt = db + MP_USED(b) - 1;
 
 	while (last < 3)
-		SETUP(mp_int_init_size(TEMP(last), 2 * umu), last);
+	{
+		SETUP(mp_int_init_size(TEMP(last), 4 * umu), last);
+		ZERO(MP_DIGITS(TEMP(last - 1)), MP_ALLOC(TEMP(last - 1)));
+	}
 
 	(void) mp_int_set_value(c, 1);