avcodec/hevc: reset long_term_rps.nb_refs for IDR

For mv-hevc, the second layer of IDR frame can be a P slice.
long_term_rps wasn't been reset before the patch, which leading to
ff_hevc_frame_nb_refs return incorrect result.

This fix decoding failure for samples from Pico VR.

libavcodec/hevc/hevcdec.c

@@ -917,6 +917,7 @@ static int hls_slice_header(SliceHeader *sh, const HEVCContext *s, GetBitContext
             sh->short_term_ref_pic_set_size     = 0;
             sh->short_term_rps                  = NULL;
             sh->long_term_ref_pic_set_size      = 0;
+            sh->long_term_rps.nb_refs           = 0;
             sh->slice_temporal_mvp_enabled_flag = 0;
         }