hw/mips/boston: Initialize g_autofree pointers

Fixes compilation due to false positives with -Werror:

  In file included from /usr/include/glib-2.0/glib.h:114,
                   from qemu/src/include/glib-compat.h:32,
                   from qemu/src/include/qemu/osdep.h:144,
                   from ../src/hw/mips/boston.c:20:
  In function ‘g_autoptr_cleanup_generic_gfree’,
      inlined from ‘boston_mach_init’ at ../src/hw/mips/boston.c:790:52:
  /usr/include/glib-2.0/glib/glib-autocleanups.h:28:3: error: ‘dtb_load_data’ may be used uninitialized [-Werror=maybe-uninitialized]
     28 |   g_free (*pp);
        |   ^~~~~~~~~~~~
  ../src/hw/mips/boston.c: In function ‘boston_mach_init’:
  ../src/hw/mips/boston.c:790:52: note: ‘dtb_load_data’ was declared here
    790 |             g_autofree const void *dtb_file_data, *dtb_load_data;
        |                                                    ^~~~~~~~~~~~~
  In function ‘g_autoptr_cleanup_generic_gfree’,
    inlined from ‘boston_mach_init’ at ../src/hw/mips/boston.c:790:36:
  /usr/include/glib-2.0/glib/glib-autocleanups.h:28:3: error: ‘dtb_file_data’ may be used uninitialized [-Werror=maybe-uninitialized]
     28 |   g_free (*pp);
        |   ^~~~~~~~~~~~
  ../src/hw/mips/boston.c: In function ‘boston_mach_init’:
  ../src/hw/mips/boston.c:790:36: note: ‘dtb_file_data’ was declared here
    790 |             g_autofree const void *dtb_file_data, *dtb_load_data;
        |                                    ^~~~~~~~~~~~~
  cc1: all warnings being treated as errors

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220605151908.30566-1-shentey@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>