RIS Encoder might encounter non-digits for EP

Problem/Motivation

I got an RIS record that did not have a digit in the EP field, it had a plus sign (+)

Steps to reproduce

Try to import the attached RIS file using the import form with RIS format setting.

Proposed resolution

accompany the key test with an integer test when storing the max value for EP within loop

Remaining tasks

I have attached a naive patch to point out where the change could be made

User interface changes

N/A

API changes

N/A

Data model changes

None that I know of, unless there are more typed data checks elsewhere!

Thank you!