In generateTrace we check that event timestamp is within the interesting range.
Then later in traceContext.time we double check event time.
However, for some events (e.g. emitSlice) we convert time of ev.Link (slice end) rather than ev itself (slice begin).
Slice end can be outside of the interesting time range, and so traceContext.time crashes.
Remove the check in traceContext.time, check in generateTrace loop is sufficient.

Change-Id: If94e93b5653c5816c0a8dcdd920f15df97616835
Reviewed-on: https://go-review.googlesource.com/11100Reviewed-by: Andrew Gerrand <adg@golang.org>