check event have ftrace_event_call associated first in e.tostring()

Since current raw kprobe fired event don't have ftrace_event_call
associated, so cannot use e.tostring() for kprobe event, check it.

(In future, kprobe event will use kprobe-tracer, then kprobe event
will also have string presentation)
Merge branch 'master' of http://github.com/ktap/ktap