fbdev: core: fbcvt: avoid division by 0 in fb_cvt_hperiod()

In fb_find_mode_cvt(), iff mode->refresh somehow happens to be 0x80000000,
cvt.f_refresh will become 0 when multiplying it by 2 due to overflow. It's
then passed to fb_cvt_hperiod(), where it's used as a divider -- division
by 0 will result in kernel oops. Add a sanity check for cvt.f_refresh to
avoid such overflow...

Found by Linux Verification Center (linuxtesting.org) with the Svace static
analysis tool.

Fixes: 96fe6a2109db ("[PATCH] fbdev: Add VESA Coordinated Video Timings (CVT) support")
Signed-off-by: Sergey Shtylyov <[e-mail]>
Signed-off-by: Helge Deller <[e-mail]>

drivers/video/fbdev/core/fbcvt.c

···
 	cvt.f_refresh = cvt.refresh;
 	cvt.interlace = 1;
 
-	if (!cvt.xres || !cvt.yres || !cvt.refresh) {
+	if (!cvt.xres || !cvt.yres || !cvt.refresh || cvt.f_refresh > INT_MAX) {
 		printk(KERN_INFO "fbcvt: Invalid input parameters\n");
 		return 1;
 	}