fbdev: hyperv_fb: fix uninitialized local variable use

When CONFIG_SYSFB is disabled, the hyperv_fb driver can now run into
undefined behavior on a gen2 VM, as indicated by this smatch warning:

drivers/video/fbdev/hyperv_fb.c:1077 hvfb_getmem() error: uninitialized symbol 'base'.
drivers/video/fbdev/hyperv_fb.c:1077 hvfb_getmem() error: uninitialized symbol 'size'.

Since there is no way to know the actual framebuffer in this configuration,
just return an allocation failure here, which should avoid the build
warning and the undefined behavior.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/202311070802.YCpvehaz-lkp@intel.com/


Fixes: a07b50d8 ("hyperv: avoid dependency on screen_info")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Helge Deller <deller@gmx.de>