If you play
Radagast and move, then he dies, you can still move the amount that
Radagast had modified the move limit to.
Radagast is played - move limit +1
Radagast dies - move limit still +1 for that turn, the next fellowship phase it will return to normal
This rule applies universally. Another situation would be
Theoden, The Renowned with Theodred out. The move limit is +1 when Theoden can spot Theodred. If either die, the move limit is still +1 for that turn. Again,
Riding Like The Wind is out, gets discarded, its move limit bonus applies until the end of the turn.
Silly I know, but since when did Decipher make sense?