fix ECH parameter overflow bug.

[ttdoda]

ECH parameter overflow bug was found in putty.
It seems that mintty has the same problem.

c.f. http://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/vuln-ech-overflow.html

[mintty]

Thanks; I guess I'd rather fix it like this:

   int arg0 = term.csi_argv[0], arg1 = term.csi_argv[1];
+  if (arg0 < 0)
+    arg0 = 0;
+  if (arg1 < 0)
+    arg1 = 0;
   int arg0_def1 = arg0 ?: 1;  // first arg with default 1

so to handle it like a signed positive value-range overflow.

[mintty]

Added multiple guards.

[mintty]

Fixed in 2.2.2.