If the analyzed program does not read any memory, or the reading
memory accesses are filtered out in the import step (import-trace -m),
the trace table may hold no entries for the program. This commit
makes sure the SamplingPruner and FESamplingPruner deal properly with
this situation.
Change-Id: I6bb5da23f345fa97cf8ab0b688cce5d00945249a
67f1be840f