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

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

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.