Set last_playing to NULL when we don't have up to date information

Set last_playing to NULL when we don't have up to date information
about the playing track (including, importantly, just after pausing).
column_length() returns NULL in this case, and ql_update_row() leaves
cells that have a NULL value along.

The effect is that the played so far indicator is left as it is until
we're sure what the right value is.  Usually in fact it will already
be right, so we don't even lie; when it isn't, at least it jumps
around less before reaching the right value.

http://code.google.com/p/disorder/issues/detail?id=26