[ntuple] Fix decodeZigzag64 for SplitInt64 columns

[undefined-panda]

DataView has no getUint64 or setInt64 methods, so reading a
SplitInt64 column raised a TypeError. Use the BigInt variants
getBigUint64 and setBigInt64.

[linev]

Thanks for fixing it.
Did you observer bug with real file?

[undefined-panda]

Yes, I used a file which is created by the validation suite of ROOT. The SplitInt64 field raised an error in decodeZigZag64. Additionally, to avoid precision loss for BigInt entries, I had to remove Number() in rntuple.mjs line 909 and 917.

Both the .root and .mjs file used are in the zip-folder:
read_integer.zip

[linev]

Conversion into number in lines 909 and 917 done because of limitation in TTree drawing functionality in JSROOT.
Before I can remove this conversion one need to resolve it.