Handle EOF explicitly in incident_helper

getLine returns -1 when it reaches EOF, and currently incident_helper
does not differentiate between EOF and an actual error when calling
getLinefrom file. So incident_helper throws an error when it reaches
EOF, and this is not good. This change separates these two cases.

Bug: 149895022
Bug: 150145113
Test: incident_helper_test
Change-Id: Ic5e67f8f9e55719c17d624718b730981c4249da1