trace_buffer: fix wrap corner case

If the functor reading the first entry after wrap-around returned false,
the wrapping was not applied successfully.

genodelabs/genode#4430

repos/base/include/base/trace/buffer.h

@@ -124,6 +124,8 @@ class Genode::Trace::Buffer
 
 			public:
 
+				Entry() : _entry(0) { };
+
 				size_t      length() const { return _entry->len; }
 				char const *data()   const { return _entry->data; }