segmentation fault in darshan-convert

Reported by Mahzad Khoshlessan with reproducer on Darshan-users mailing list:

https://lists.mcs.anl.gov/pipermail/darshan-users/2019-January/000522.html

Bug report is for darshan-summary-per-file.sh, but the problem is actually in darshan-convert (which is called by the per-file script). The same log works fine with darshan-parser and darshan-job-summary.pl.