audio_frame2::change_ch_count: fix CID 480257

the behavior is now very slightly modified but effectively the same,
eg. change 1 to 3 channels copies now channel 0 to 1 and then 1 to 2
(before it copies 0 to both 1 and 2).

Author: MartinPulec

src/audio/types.cpp

@@ -347,8 +347,8 @@ void audio_frame2::change_ch_count(int ch_count)
                 return;
         }
 
-        const channel &last_ch = channels.at(channels.size() - 1);
         for (unsigned i = channels.size(); i < (unsigned) ch_count; ++i) {
+                const channel &last_ch = channels.at(channels.size() - 1);
                 std::unique_ptr<char[]> data =
                     std::unique_ptr<char[]>(new char[last_ch.len]);
                 memcpy(data.get(), last_ch.data.get(), last_ch.len);