[gdbserver] Fix trace-buffer-size.exp FAILs.

[gdbserver] Fix trace-buffer-size.exp FAILs.

I'm seeing trace-buffer-size.exp failing (with gdbserver):

  (gdb) PASS: gdb.trace/trace-buffer-size.exp: tstatus check 2
  show trace-buffer-size 4
  Requested size of trace buffer is 4.
  (gdb) PASS: gdb.trace/trace-buffer-size.exp: show trace buffer size
  set trace-buffer-size -1
  memory clobbered past end of allocated block
  Remote connection closed
  (gdb) FAIL: gdb.trace/trace-buffer-size.exp: set trace buffer size 2
  set trace-buffer-size unlimited
  (gdb) PASS: gdb.trace/trace-buffer-size.exp: set trace-buffer-size unlimited

That "memory clobbered past end of allocated block" is mcheck triggering.

Valgrind shows:

  ==23624== Invalid write of size 1
  ==23624==    at 0x418DD8: clear_trace_buffer (tracepoint.c:1443)
  ==23624==    by 0x418F3A: init_trace_buffer (tracepoint.c:1497)
  ==23624==    by 0x41D95B: cmd_bigqtbuffer_size (tracepoint.c:4061)
  ==23624==    by 0x41DEEC: handle_tracepoint_general_set (tracepoint.c:4193)

clear_trace_buffer does:

  static void
  clear_trace_buffer (void)
  {
    trace_buffer_start = trace_buffer_lo;
    trace_buffer_free = trace_buffer_lo;
    trace_buffer_end_free = trace_buffer_hi;
    trace_buffer_wrap = trace_buffer_hi;
    /* A traceframe with zeroed fields marks the end of trace data.  */
    ((struct traceframe *) trace_buffer_free)->tpnum = 0;
    ((struct traceframe *) trace_buffer_free)->data_size = 0;
    traceframe_read_count = traceframe_write_count = 0;
    traceframes_created = 0;
  }

And the tpnum+data_size fields are over 4 bytes...  This fixes it by
ensuring we allocate space at least for an EOB.  We have code
elsewhere that relies on the EOB being present (like e.g.,
find_traceframe), so this seems simplest.

gdb/gdbserver/
2013-09-02  Pedro Alves  <palves@redhat.com>

	* tracepoint.c (TRACEFRAME_EOB_MARKER_SIZE): New macro.
	(init_trace_buffer): Ensure at least TRACEFRAME_EOB_MARKER_SIZE is
	allocated.
	(trace_buffer_alloc): Use TRACEFRAME_EOB_MARKER_SIZE.