Prevent index out of array bounds exception

[Siliconrob]

Any updates on when this might get merged + released?

Originally posted by @mason-mcglothlin in https://github.com/JanKallman/EPPlus/pull/475#issuecomment-561720543

short -> int index change : Requesting a merge of this non breaking change to master so that next nuget package publish will have this please.