ntdll: Return error if the signal index equals the size of the array.

dlls/ntdll/signal_i386.c

@@ -1436,7 +1436,7 @@ static int set_handler( int sig, int have_sigaltstack, void (*func)() )
  */
 int __wine_set_signal_handler(unsigned int sig, wine_signal_handler wsh)
 {
-    if (sig > sizeof(handlers) / sizeof(handlers[0])) return -1;
+    if (sig >= sizeof(handlers) / sizeof(handlers[0])) return -1;
     if (handlers[sig] != NULL) return -2;
     handlers[sig] = wsh;
     return 0;