fbmon: prevent division by zero in fb_videomode_from_videomode()

The expression htotal * vtotal can have a zero value on
overflow. It is necessary to prevent division by zero like in
fb_var_to_videomode().

Found by Linux Verification Center (linuxtesting.org) with Svace.
Signed-off-by: Roman Smirnov <r.smirnov@omp.ru>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Helge Deller <deller@gmx.de>

drivers/video/fbdev/core/fbmon.c

@@ -1311,7 +1311,7 @@ int fb_get_mode(int flags, u32 val, struct fb_var_screeninfo *var, struct fb_inf
 int fb_videomode_from_videomode(const struct videomode *vm,
 				struct fb_videomode *fbmode)
 {
-	unsigned int htotal, vtotal;
+	unsigned int htotal, vtotal, total;
 
 	fbmode->xres = vm->hactive;
 	fbmode->left_margin = vm->hback_porch;
@@ -1344,8 +1344,9 @@ int fb_videomode_from_videomode(const struct videomode *vm,
 	vtotal = vm->vactive + vm->vfront_porch + vm->vback_porch +
 		 vm->vsync_len;
 	/* prevent division by zero */
-	if (htotal && vtotal) {
-		fbmode->refresh = vm->pixelclock / (htotal * vtotal);
+	total = htotal * vtotal;
+	if (total) {
+		fbmode->refresh = vm->pixelclock / total;
 	/* a mode must have htotal and vtotal != 0 or it is invalid */
 	} else {
 		fbmode->refresh = 0;