Handle unlikely case for qemu perfetto stats recording.

Avoid creating histogram buckets starting at 0 when pause is called
but no instructions have been executed yet.

trace_extra/trace_stats.cc

@@ -79,7 +79,9 @@ qemu_stats::pause(perfetto::Track &track, uint64_t pc)
     if (paused_)
         return;
     paused_ = true;
-    assert(pc_range_start_ != 0 && "PC bb start was not updated");
+    /* Nothing to flush */
+    if (pc_range_start_ == 0)
+        return;
     assert(pc != 0 && "pause PC can not be zero");
     /* Treat this as the end of the block, otherwise the current PC slice will be lost. */
     bb_hit_map_ += std::make_pair(addr_range::right_open(pc_range_start_, pc), icount_);