Divide by zero exception in jitter buffer with prefetching mechanism enabled (thanks Roman Puls for the report)

[nanang]

There is bug in jb prefetching mechanism, the prefetch condition may cause unexpected JB state switches that will disturb burst level calculation, i.e: when JB empty condition occurs in initializing state, the state will be immediately switched to prefetching, and after the prefetch processing done, the state will be directly set to processing (instead of initializing). The expected behaviour is that JB state switch from initializing to processing is only done by jb_update().

This causes the following division by zero exception:

Program received signal SIGFPE, Arithmetic exception.
[Switching to Thread 0xb73a5b90 (LWP 7148)]
0x08184372 in pjmedia_jbuf_put_frame2 (jb=0x8323cf8, frame=0x826f86c, frame_size=80, bit_info=0, frame_seq=21, discarded=0xb73a502c)
   at ../src/pjmedia/jbuf.c:763
763                 burst_factor = cur_size / burst_level;
(gdb)

[nanang]

(In [3568]) Fix #1258: Fixed bug in jitter buffer, i.e: unexpected state switches when prefetch mechanism is active, by separating prefetch mechanism from JB state.