Bug 1729: Security fixes from PuTTY 0.71 - vuln-rng-reuse

https://winscp.net/tracker/1729
https://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/vuln-rng-reuse.html
(cherry picked from commit 8b0202f6b2de1de15bb237b9aa549188a11cb4ae)

Source commit: 981dc96f691b07edf469a2b690b594c621ba8b3a

source/putty/sshrand.c

@@ -240,6 +240,8 @@ void random_add_noise(void *noise, int length)
 	length -= HASHINPUT - pool.incomingpos;
 	SHATransform((word32 *) pool.incoming, (word32 *) pool.incomingb);
 	for (i = 0; i < HASHSIZE; i++) {
+	    if (pool.poolpos >= POOLSIZE)
+		pool.poolpos = 0;
 	    pool.pool[pool.poolpos++] ^= pool.incoming[i];
 	    if (pool.poolpos >= POOLSIZE)
 		pool.poolpos = 0;