validate_failures.py: Fix parsing for DejaGNU sumfiles

Commit a0d9054bc6cf ("[contrib] validate_failures.py: Support json
results") changed the for loop's iteration variable in ParseSummary but
forgot to set the line variable in case the summary isn't a JSON file.

Restore previous behaviour when the input file isn't JSON.

Change-Id: Iad68b7221594b9aba82c02663d067fb1a58ef1a5
1 file changed