prevent num_bits_left underflow in ps_data extension parsing

[netliomax25-code]

1. ps_extension() decodes a variable amount of IPD/OPD Huffman data and does not stop at the advertised ps_extension_size, so it can read more bits than num_bits_left.
2. num_bits_left is a uint16_t, so num_bits_left -= ps_extension(...) wraps near 65535 and the while (num_bits_left > 7) loop keeps re-entering ps_extension until the bitstream runs out, consuming data that belongs to the rest of the frame (15726 iterations instead of 4 on a crafted HE-AACv2 frame here).

Clamp num_bits_left to zero when a sub-extension reads past the remaining budget instead of subtracting. Reachable from the SBR extension of a channel-pair element in the default build; valid HE-AACv2 output is unchanged.