hw/mips/boston: Check for error return from boston_fdt_filter()

hw/mips/boston: Check for error return from boston_fdt_filter()

The function boston_fdt_filter() can return NULL on errors (in which
case it will print an error message).  When we call this from the
non-FIT-image codepath, we aren't checking the return value, so we
will plough on with a NULL pointer, and segfault in fdt_totalsize().
Check for errors here.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20250206151214.2947842-4-peter.maydell@linaro.org