Land rapid7#7290, env var check for WinSCP module

Author: wvu

modules/post/windows/gather/credentials/winscp.rb

@@ -114,6 +114,14 @@ def run
     end
     env = get_envs('APPDATA', prog_files_env, 'USERNAME')
 
+    if env['APPDATA'].nil?
+      fail_with(Failure::Unknown, 'Target does not have environment variable APPDATA')
+    elsif env[prog_files_env].nil?
+      fail_with(Failure::Unknown, "Target does not have environment variable #{prog_files_env}")
+    elsif env['USERNAME'].nil?
+      fail_with(Failure::Unknown, 'Target does not have environment variable USERNAME')
+    end
+
     user_dir = "#{env['APPDATA']}\\..\\.."
     user_dir << "\\.." if user_dir.include?('Users')