fix(Mp42Hls): reject E-AC-3 sample descriptions with no usable dec3 atom

[AetherAI3]

Closes #1043.

The E-AC-3 branch of mp42hls's audio-stream setup does

AP4_Dec3Atom* dec3 = AP4_DYNAMIC_CAST(AP4_Dec3Atom,
    sample_description->GetDetails().GetChild(AP4_ATOM_TYPE_DEC3));
...
if (dec3->GetSubStreams()[0].acmod == 0) { ... }

with no check that the cast returned non-NULL, and no check that the substreams array is non-empty. A crafted MP4 whose sample description advertises EC-3 (AP4_SAMPLE_FORMAT_EC_3) but lacks a well-formed dec3 child atom sends dec3 to NULL, and dec3->GetSubStreams()[0].acmod faults reading offset 12 into the zero page (@sigdevel's PoC on the mp42hls/5 sample). Even if dec3 is present, its substreams array can be empty — GetSubStreams()[0] then binds a reference to a null pointer via AP4_Array::operator[], and the deref faults at the same offset one line further down.

Guard the block right after the cast: reject the sample if dec3 is NULL or its substreams array has no entries, print an error matching the surrounding style (ERROR: E-AC-3 sample description is missing a usable dec3 atom, same shape as the sample_description == NULL message a few dozen lines below) and goto end to run the existing teardown. Valid E-AC-3 inputs always carry a dec3 atom with at least one substream, so the guard falls through and nothing observable changes on the happy path.

Reported-by: @sigdevel — thanks for the report and reproducer.