fix: limit memory usage in reading invalid subs box

[eric]

Found this when fuzzing:

ROUTINE ======================== github.com/Eyevinn/mp4ff/mp4.DecodeSubsSR in /Users/eric/src/mp4ff/mp4/subs.go
  255.17GB   255.17GB (flat, cum)   100% of Total
         .          .     69:func DecodeSubsSR(hdr BoxHeader, startPos uint64, sr bits.SliceReader) (Box, error) {
         .          .     70:	versionAndFlags := sr.ReadUint32()
         .          .     71:	version := byte(versionAndFlags >> 24)
         .          .     72:
         .          .     73:	b := SubsBox{
         .          .     74:		Version: version,
         .          .     75:		Flags:   versionAndFlags & flagsMask,
         .          .     76:	}
         .          .     77:	entryCount := sr.ReadUint32()
         .          .     78:	for i := uint32(0); i < entryCount; i++ {
         .          .     79:		e := SubsEntry{}
         .          .     80:		e.SampleDelta = sr.ReadUint32()
         .          .     81:		subsampleCount := sr.ReadUint16()
         .          .     82:		for j := uint16(0); j < subsampleCount; j++ {
         .          .     83:			ss := SubsSample{}
         .          .     84:			if version == 1 {
         .          .     85:				ss.SubsampleSize = sr.ReadUint32()
         .          .     86:			} else {
         .          .     87:				ss.SubsampleSize = uint32(sr.ReadUint16())
         .          .     88:			}
         .          .     89:			ss.SubsamplePriority = sr.ReadUint8()
         .          .     90:			ss.Discardable = sr.ReadUint8()
         .          .     91:			ss.CodecSpecificParameters = sr.ReadUint32()
    1.21MB     1.21MB     92:			e.SubSamples = append(e.SubSamples, ss)
         .          .     93:		}
  255.17GB   255.17GB     94:		b.Entries = append(b.Entries, e)
         .          .     95:	}
         .          .     96:	return &b, sr.AccError()
         .          .     97:}
         .          .     98:
         .          .     99:// Type - return box type

[tobbee]

LGTM! Thanks!