Fix #4246 - Access payload_exe information correctly

This fixes an undef method 'payload_exe' error. We broke this when
all modules started using Msf::Exploit::CmdStager as the only source
to get a command stager payload. The problem with that is "payload_exe"
is an accessor in CmdStagerTFTP, not in CmdStager, so when the module
wants to access that, we trigger the undef method error.

To be exact, this is the actual commit that broke it:
7ced5927d8807e385c0375a25ef71aef404d3036

Fix #4246
Land #4321, @wchen-r7's fixes #4246 ms01_026_dbldecode undef method