setup: fix higher-than

in rare cases, no pattern would be recognized as a valid bitrate for "higher-than"

lib/setup/destination

@@ -451,7 +451,6 @@ setupDestination() {
 			echo "Invalid frequency value: $value" >&2
 			comeagain
 		fi
-		unset expr
 	}
 	comeagain
 	if [ -n "${destinationfrequency["$destination"]}" ] \
@@ -492,4 +491,5 @@ setupDestination() {
 	fi
 	destinationmaxbps[$destination]="$value"
 	unset regen
+	unset expr
 }