[MAD Decoder] Fix crash on invalid files

The properties function should not be dereferencing an invalid index
into the layer codec name array if layer is not set to 1 through 3. This
could happen if, for instance, an MP3 file has an invalid ID3v2 tag.

Signed-off-by: Christopher Snowhill <kode54@gmail.com>

Plugins/MAD/MADDecoder.m

@@ -756,6 +756,7 @@
 }
 
 - (NSDictionary *)properties {
+	if(layer < 1 || layer > 3) return nil;
 	const NSString *layers[3] = { @"MP1", @"MP2", @"MP3" };
 	return @{ @"channels": [NSNumber numberWithInt:channels],
 		      @"bitsPerSample": [NSNumber numberWithInt:32],