fbdev: tdfxfb: avoid divide-by-zero on FBIOPUT_VSCREENINFO

fbdev: tdfxfb: avoid divide-by-zero on FBIOPUT_VSCREENINFO

commit 8f98b81fe011e1879e6a7b1247e69e06a5e17af2 upstream.

Much like commit 19f953e74356 ("fbdev: fb_pm2fb: Avoid potential divide
by zero error"), we also need to prevent that same crash from happening
in the udlfb driver as it uses pixclock directly when dividing, which
will crash.

Cc: Helge Deller <deller@gmx.de>
Assisted-by: gregkh_clanker_t1000
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>