Fix overflow, from upstream
https://sourceforge.net/p/crossfire/crossfire-client/ci/4c98710209e39e379fccd6aa9db96978df21fe4b/

> inlined from ‘cs_print_string’ at common/newsocket.c:260:15:
> /usr/include/bits/stdio2.h:100:10:
> warning: ‘__builtin___vsnprintf_chk’ specified bound 256 exceeds destination size 254 [-Wstringop-overflow=]

--- a/common/newsocket.c
+++ b/common/newsocket.c
@@ -20,6 +20,7 @@
 
 #include "client.h"
 
+#include <assert.h>
 #include <errno.h>
 
 #include "script.h"
@@ -257,12 +258,9 @@
 
     SockList_Init(&sl, buf);
     va_start(args, str);
-    sl.len += vsnprintf((char*)sl.buf + sl.len, MAX_BUF-sl.len, str, args);
+    sl.len += vsnprintf((char*)sl.buf + sl.len, MAX_BUF-sl.len-3, str, args); // need 2 bytes for length, 1 for null termination
     va_end(args);
-    // Adjust sl.len to account for when we overflow the buffer.
-    if (sl.len > MAX_BUF - 3) {
-        sl.len = MAX_BUF - 3;
-    }
+    assert(sl.len <= MAX_BUF - 3);
 
     script_monitor_str((char*)sl.buf);