fbmon: prevent division by zero in fb_videomode_from_videomode()

In the Linux kernel, the following vulnerability has been resolved: 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.