DiskReader: always reset file_sample[AUDIO] after an overwrite

In theory we only need to do this if the use of a loop for a given overwrite differs
from the previous refill or overwrite. However, keeping track of this is hard, and
this way effectively enforces the notion that if we do the arithmetic correct,
for cases where there's no change in the use of a loop location, this ends up
being a no-op (i.e. we are resetting it back to its current value)

libs/ardour/disk_reader.cc

@@ -748,9 +748,7 @@ DiskReader::overwrite_existing_audio ()
 		}
 	}
 
-	if (_loop_location) {
-		file_sample[DataType::AUDIO] = start;
-	}
+	file_sample[DataType::AUDIO] = start;
 
 	return ret;
 }