patch 8.2.5069: various warnings from clang on MS-Windows

Problem:    Various warnings from clang on MS-Windows.
Solution:   Fix the code to avoid the warnings. (Yegappan Lakshmanan,
            closes #10538)
diff --git a/src/gui_w32.c b/src/gui_w32.c
index 0bac12a..3e1addb 100644
--- a/src/gui_w32.c
+++ b/src/gui_w32.c
@@ -4153,7 +4153,8 @@
 {
     int		button;
     win_T	*wp;
-    int		modifiers, kbd_modifiers;
+    int		modifiers = 0;
+    int		kbd_modifiers;
     int		zDelta = GET_WHEEL_DELTA_WPARAM(wParam);
     POINT	pt;
 
@@ -4213,7 +4214,7 @@
     pt.y = GET_Y_LPARAM(lParam);
     ScreenToClient(s_textArea, &pt);
 
-    gui_send_mouse_event(button, pt.x, pt.y, FALSE, kbd_modifiers);
+    gui_send_mouse_event(button, pt.x, pt.y, FALSE, modifiers);
 }
 
 #ifdef USE_SYSMENU_FONT