video: fbdev: i740fb: Error out if 'pixclock' equals zero

The userspace program could pass any values to the driver through
ioctl() interface. If the driver doesn't check the value of 'pixclock',
it may cause divide error.

Fix this by checking whether 'pixclock' is zero in the function
i740fb_check_var().

The following log reveals it:

divide error: 0000 [#1] PREEMPT SMP KASAN PTI
RIP: 0010:i740fb_decode_var drivers/video/fbdev/i740fb.c:444 [inline]
RIP: 0010:i740fb_set_par+0x272f/0x3bb0 drivers/video/fbdev/i740fb.c:739
Call Trace:
    fb_set_var+0x604/0xeb0 drivers/video/fbdev/core/fbmem.c:1036
    do_fb_ioctl+0x234/0x670 drivers/video/fbdev/core/fbmem.c:1112
    fb_ioctl+0xdd/0x130 drivers/video/fbdev/core/fbmem.c:1191
    vfs_ioctl fs/ioctl.c:51 [inline]
    __do_sys_ioctl fs/ioctl.c:874 [inline]

Signed-off-by: Zheyu Ma <zheyuma97@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>
