avcodec/hevcdec: fix stat_coeff save/load for persistent_rice_adaptation_enabled_flag

avcodec/hevcdec: fix stat_coeff save/load for persistent_rice_adaptation_enabled_flag

It's required by the 9.3.1 TableStatCoeff* section.

Following clips have this feature:
WPP_HIGH_TP_444_8BIT_RExt_Apple_2.bit
Bitdepth_A_RExt_Sony_1.bin
Bitdepth_B_RExt_Sony_1.bin
EXTPREC_HIGHTHROUGHPUT_444_16_INTRA_10BIT_RExt_Sony_1.bit
EXTPREC_HIGHTHROUGHPUT_444_16_INTRA_12BIT_RExt_Sony_1.bit
EXTPREC_HIGHTHROUGHPUT_444_16_INTRA_8BIT_RExt_Sony_1.bit
EXTPREC_MAIN_444_16_INTRA_10BIT_RExt_Sony_1.bit
EXTPREC_MAIN_444_16_INTRA_12BIT_RExt_Sony_1.bit
EXTPREC_MAIN_444_16_INTRA_8BIT_RExt_Sony_1.bit
WPP_AND_TILE_10Bit422Test_HIGH_TP_444_10BIT_RExt_Apple_2.bit
WPP_AND_TILE_AND_CABAC_BYPASS_ALIGN_0_HIGH_TP_444_14BIT_RExt_Apple_2.bit
WPP_AND_TILE_AND_CABAC_BYPASS_ALIGN_1_HIGH_TP_444_14BIT_RExt_Apple_2.bit
WPP_AND_TILE_HIGH_TP_444_8BIT_RExt_Apple_2.bit

you can download them from:
https://www.itu.int/wftp3/av-arch/jctvc-site/bitstream_exchange/draft_conformance/RExt/

Signed-off-by: Xu Guangxin <oddstone@gmail.com>
Signed-off-by: Linjie Fu <linjie.justin.fu@gmail.com>