pki: Reset variable so error handling works properly

If we jump to `end` without this we crash (not necessarily visibly) due
to a double free and the actual error message is not printed.

src/pki/commands/issue.c

@@ -406,6 +406,7 @@ static int issue()
 		goto end;
 	}
 	public->destroy(public);
+	public = NULL;
 
 	if (hex)
 	{