Check for null ferr

fout is double checked for nullptr instead of ferr which forces
to call fclose on null ferr.

Test: atest incidentd_test
Test: m incidentd_service_fuzzer && adb sync data && adb shell /data/fuzz/x86_64/incidentd_service_fuzzer/incidentd_service_fuzzer
with clusterfuzz testcase
Bug: 283453191

Change-Id: I3a4912cf70f4f32cb66fb870febbbfff1a7c6adc