exception with edbg:file() when trace file is empty

[hshibuya-dd]

When I haven't started any trace yet, I get:

> edbg:file().       
** exception error: bad argument
     in function  binary_to_term/1
        called as binary_to_term(<<>>)
     in call from edbg_tracer:file/2 (src/edbg_tracer.erl, line 356)