silence clang warning by using local variable.
'isc_commandline_index' is a global variable so it can theoretically change result between if expressions. Save 'argv[isc_commandline_index]' to local variable 'arg1' and use 'arg1 == NULL' in if expressions instead of 'argc < isc_commandline_index + 1'. This allows clang to correctly determine what code is reachable.