softfloat: Silence a warning at -Og

softfloat: Silence a warning at -Og

   ../src/util/softfloat.c: In function ‘_mesa_shift_right_jam_m’:
   ../src/util/softfloat.c:432:16: warning: ‘tmp’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     432 |         *tmp++ = 0;

You could actually hit this if you called _mesa_shift_right_jam_m with
size_words = 0 and dist < 32. Not that you'd _do_ that, but. In this
case do nothing instead of writing through an uninitialized pointer.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8724>