Update H5Zlz4.c

[Mark Kittisopikul]

The H5Zlz4.c in this repository has a few issues that have been resolved here:

https://github.com/nexusformat/HDF5-External-Filter-Plugins/blob/master/LZ4/src/H5Zlz4.c

In particular:

• Fix filter not compressing with LZ4 version>1.3.0
  • nexusformat/HDF5-External-Filter-Plugins@d48f960#diff-5bbc145a3b14bd0892bda69fbbc14c5c1de507209ec680df323dde685fcffb54
• Use H5allocate_memory() and H5free() rather than malloc() and free() …
  • nexusformat/HDF5-External-Filter-Plugins@1e1cb31#diff-5bbc145a3b14bd0892bda69fbbc14c5c1de507209ec680df323dde685fcffb54

[Thomas VINCENT]

I've asked upstream to tag a version on their repo to have a clear reference: nexusformat/HDF5-External-Filter-Plugins#19 .
I'll update and pin a commit rather than a release tag if none is set.