Check for null system device

Check for null device in gsi service and return invalid fd
when device is null. Adding checks for invalid fd.

Test: atest vts_gsi_boot_test
Test: m gsi_service_fuzzer && adb sync data && adb shell /data/fuzz/x86_64/gsi_service_fuzzer/gsi_service_fuzzer -runs=10000
Bug: 313514072
Change-Id: I89bda8a48267e98f180d273d338aac6c869f9876
2 files changed