mux: extradata: fix `hxxx_helper` leak

The builder wasn't releasing the helper's internal leading to several leaks. This was likely forgotten.

Caught with ASAN:

Direct leak of 516 byte(s) in 1 object(s) allocated from:
    #0 0x7fa012fddcd7 in calloc (/lib64/libasan.so.8+0xd8cd7)
    #1 0x7f9fe032dd15 in hevc_decode_sps modules/packetizer/hevc_nal.c:929
    #2 0x7f9fe0371883 in hevc_helper_parse_nal modules/codec/hxxx_helper.c:315
    #3 0x7f9fe0374131 in hxxx_helper_process_nal modules/codec/hxxx_helper.c:516
    #4 0x7f9fe0374532 in hxxx_helper_process_buffer modules/codec/hxxx_helper.c:535
    #5 0x7f9fe0354661 in hxxx_extradata_builder_Feed modules/mux/extradata.c:174
    #6 0x7f9fe0355701 in mux_extradata_builder_Feed modules/mux/extradata.c:263
    #7 0x7f9fe02ed827 in BlockDequeue modules/mux/mp4/mp4.c:616
    #8 0x7f9fe02fbe7f in MuxFrag modules/mux/mp4/mp4.c:1458