rtp_engine: Failed assertion and wrong name given for codec

Fixed an assert check that would trigger when the passed in value was negative.
The negative value was being cast to an unsigned value. This resulted in the
check failing.

Also fixed another problem when loading formats in the engine. When setting the
mime type the format's name was being passed in instead of the codec's name.

Change-Id: I1a201cd419ba4d8e9a40d337e36b6fbe1737192c