Added an assertion to avoid surprises if you call the not-yet-implemented MP3AudioFormat::createWriterFor() method

modules/juce_audio_formats/codecs/juce_MP3AudioFormat.cpp

@@ -3147,6 +3147,7 @@ AudioFormatWriter* MP3AudioFormat::createWriterFor (OutputStream*, double /*samp
                                                     unsigned int /*numberOfChannels*/, int /*bitsPerSample*/,
                                                     const StringPairArray& /*metadataValues*/, int /*qualityOptionIndex*/)
 {
+    jassertfalse; // not yet implemented!
     return nullptr;
 }