kernel-shark: Only warn in file dialog if file exists
kernel-shark: Only warn in file dialog if file exists

The warning of the file dialog will happen everytime even
if the file does not exist. Fix that.

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

bigzz/trace-cmd,bigzz/trace-cmd