root: catch segfault when converting large tables

[GraemeWatt]

The ROOT conversion of https://www.hepdata.net/record/ins1742786 fails with an HTML file containing a message "502 Bad Gateway. The server returned an invalid or incomplete response.". By running the conversion offline, most tables can be converted other than "Statistical covariance matrix" and "Systematic covariance matrix" which gave an error message like:

Fatal in <TBufferFile::AutoExpand>: Request to expand to a negative size, likely due to an integer overflow: 0x91111b20 for a max of 0x7ffffffe.
aborting
[...]
qemu: uncaught target signal 6 (Aborted) - core dumped
Aborted

I can't find anything unusual in the YAML formatting of these tables, so I think the problem is simply caused by the large size (17424 rows and 2.6 MB). If the problem cannot be resolved, at least the segfault should be caught and a suitable error message returned from the problematic line of the conversion code:

hepdata-converter/hepdata_converter/writers/root_writer.py

Line 408 in 3f0330d

graph.Write()

[GraemeWatt]

I reencountered this problem today after investigating a user report and got similar error messages from the same tables like:

[...]
    graph.Write()
cppyy.ll.SegmentationViolation: none of the 2 overloaded methods succeeded. Full details:
  int TObject::Write(const char* name = 0, int option = 0, int bufsize = 0) =>
    SegmentationViolation: segfault in C++; program state was reset
  int TObject::Write(const char* name = 0, int option = 0, int bufsize = 0) =>
    SegmentationViolation: segfault in C++; program state was reset
 *** Break *** segmentation violation
[...]