Courtesy of Igor over at the Avast Free forum.
For any Avast scan that uses a memory scan including the stand alone memory scan itself, add the following to the Exclusions setting:
*PROCESS*\cmdagent.exe
If you also use MBAM Pro i.e. real-time, also add:
*PROCESS*\mbamservice.exe
Now you can run the Avast memory scan w/o those encryted signature warnings.