Avoid segfault in playback mode if no files specified

When running the mixing module in playback mode, if the secondary input source is given an empty file list, a segfault resulted when attempting to throw an exception. This pull request fixes the segfault so that the exception is thrown.
This pull request also improves exception messages that occur on an empty file list for the secondary source when playback mode is not used.
This code bug has been there for a long time. This segfault has been observed as far back as 5_3_X.
Pull request requested in 7_0_X by David Lange.