trace-cmd: Do not return offset of empty CPU buffer
trace-cmd: Do not return offset of empty CPU buffer

If a one CPU buffer is empty, and a tracecmd_read_at is made to the
first page of the next CPU buffer, it mistakenly returns the empty
buffer because the offsets still match.

Fix this by only returning a CPU buffer if it contains something.

Signed-off-by: Steven Rostedt <43232e92d70cc7aa53504ad0397085ee47bad87f@goodmis.org>

bigzz/trace-cmd,bigzz/trace-cmd