fix AC coefficient range check

[gn-ahn]

Summary

According to the specification (RFC 9924):

• The reconstructed transform coefficients must be in the range of [-32768, 32767] ([MIN_TX_VAL, MAX_TX_VAL]).

Problem

Previously, the range check was performed on level before applying the sign bit (if (level > 32767)).
This caused a valid reconstructed AC coefficient of -32768 (level == 32768 with flag == 1) to be incorrectly rejected as OAPV_ERR_MALFORMED_BITSTREAM.

Solution

• Moved the range check after sign parsing (oapv_set_sign16).
• Validated the reconstructed signed AC coefficient (val) against [MIN_TX_VAL, MAX_TX_VAL] ([-32768, 32767]).

[kpchoi]

Good point.
LGTM