tplay: will crash if the playlist is exhausted

[juanrubio]

To reproduce, simply try to stream a folder without any CBR files (e.g. a folder where all files are ABR encoded). The initial playlist will be processed and each of the files will end up being removed. After that tplay will crash with this stack trace:

Program received signal SIGFPE, Arithmetic exception.
[Switching to Thread 0x7fffea4f2700 (LWP 7190)]
0x00000000004184bc in tiz::playlist::skip (this=0x6ef230, jump=1) at tizplaylist.cpp:259
259 current_index_ %= list_size;
(gdb) bt
#0 0x00000000004184bc in tiz::playlist::skip (this=0x6ef230, jump=1) at tizplaylist.cpp:259
#1 0x0000000000437b70 in tiz::graph::ops::do_skip (this=0x7fffe0005a70) at tizgraphops.cpp:268
#2 0x000000000046509f in operator()boost::msm::back::state_machine<tiz::graph::hsfsm::fsm_::configuring_, boost::msm::front::none, tiz::graph::hsfsm::fsm_::configuring_::probing, tiz::graph::hsfsm::fsm_::configuring_::probing> (this=, evt=..., fsm=...) at tizgraphaction.hpp:84