Fix H5Zzstd error reporting and decompressed size behavior

[mkitti]

JuliaIO/JLD2.jl#560 revealed that error reporting was not occuring when there were errors present.

1. ZSTD_getDecompressedSize can result in an error, resulting in a return value of 0.
2. Libc.malloc(0) may not return a null pointer.

Therefore we need to check the return value of ZSTD_getDecompressedSize directly before
trying to malloc.

Also improve error handing overall.

[mkitti]

Also note that ZSTD_getDecompressedSize is deprecated.