block-qdict: Fix -Werror=maybe-uninitialized build failure

block-qdict: Fix -Werror=maybe-uninitialized build failure

Building QEMU on Fedora 37 (Rawhide Prerelease) ppc64le failed with the
following error:

    $ ../configure --prefix=/usr/local/qemu-disabletcg --target-list=ppc-softmmu,ppc64-softmmu --disable-tcg --disable-linux-user
    ...
    $ make -j$(nproc)
    ...
    In file included from /root/qemu/include/qapi/qmp/qdict.h:16,
                     from /root/qemu/include/block/qdict.h:13,
                     from ../qobject/block-qdict.c:11:
    /root/qemu/include/qapi/qmp/qobject.h: In function ‘qdict_array_split’:
    /root/qemu/include/qapi/qmp/qobject.h:49:17: error: ‘subqdict’ may be used uninitialized [-Werror=maybe-uninitialized]
       49 |     typeof(obj) _obj = (obj);                                   \
          |                 ^~~~
    ../qobject/block-qdict.c:227:16: note: ‘subqdict’ declared here
      227 |         QDict *subqdict;
          |                ^~~~~~~~
    cc1: all warnings being treated as errors

Fix build failure by expanding the ternary operation.
Tested with `make check-unit` (the check-block-qdict test passed).

Signed-off-by: Murilo Opsfelder Araujo <muriloo@linux.ibm.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Hanna Reitz <hreitz@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Message-Id: <20220311221634.58288-1-muriloo@linux.ibm.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>