av.c: silence compiler warning

av.c: silence compiler warning

av.c: In function ‘Perl_av_undef’:
av.c:577:35: warning: ‘orig_ix’ may be used uninitialized in this function [-Wmaybe-uninitialized]
             PL_tmps_stack[orig_ix] = &PL_sv_undef;

The warning is bogus, as we only use the orig_ix if real is true,
and if real is true we will have set orig_ix. But it doesnt cost
much to initialize it always and shut up the compiler.