patch 9.0.2143: [security]: buffer-overflow in ex_substitute

Problem:  [security]: buffer-overflow in ex_substitute
Solution: clear memory after allocating

When allocating the new_start pointer in ex_substitute() the memory
pointer points to some garbage that the following for loop in
ex_cmds.c:4743 confuses and causes it to accessing the new_start pointer
beyond it's size, leading to a buffer-overlow.

So fix this by using alloc_clear() instead of alloc(), which will
clear the memory by NUL and therefore cause the loop to terminate
correctly.

Reported by @henices, thanks!

closes: #13596
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2143,
 /**/
     2142,
 /**/