bin/53022: gpt(8) segfaults if invoked without device specified

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>Number:         53022
>Category:       bin
>Synopsis:       gpt(8) segfaults if invoked without device specified
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    bin-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Feb 13 01:10:00 +0000 2018
>Originator:     Sevan Janiyan
>Release:        NetBSD-HEAD
>Organization:
>Environment:
>Description:
gpt(8) assumes the device has been passed when invoked & attempts to enumerated the device in src/sbin/gpt/main.c, then ploughs forward.
As a work around I've added a check in each cmd_$command() 
if (gpt == NULL)
   return usage();
which works and has the benefit of returning the relevant usage for the sub command being invoked. On the other hand, I haven't worked out how to hand things in main() to stop right at the start so I only have to deal with the issue in one place.
>How-To-Repeat:
run gpart show -a
>Fix: