q800: fix segfault with invalid MacROM

q800: fix segfault with invalid MacROM

"qemu-system-m68k -M q800 -bios /dev/null" crashes with a segfault
in q800_init().
This happens because the code doesn't check that rom_ptr() returned
a non-NULL pointer .

To avoid NULL pointer, don't allow 0 sized file and use bios_size with
rom_ptr().

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/756
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20220107105049.961489-1-laurent@vivier.eu>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>