bf16: handle invalid Nan-boxed accessing

 assume
    0x0000_0000_0000_7d2d at 0x8000_0000
    a0 = 0x8000_0000

    fld ft0, 0(a0)           <-  load 0x0000_0000_0000_7d2d to ft0,
                                 it is invalid Nanboxed

    fcvt.s.bf16  ft1, ft0    <-  read bf16 from ft0.  it should be
                                 0x7fc0 (bf16 QNaN) but not 0x7e00 (f16 QNaN)

Signed-off-by: Chih-Min Chao <chihmin.chao@sifive.com>