simpleperf: Fix cached map in OfflineUnwinder.

In system wide profiling, sometimes a sample record goes before all
mmap records of a process. Then thread.maps->version = 0 and
cached_map.map isn't created.

This is caught by simpleperf_unit_test.

Bug: none
Test: run simpleperf_unit_test.
Change-Id: I99254396d0fb22531f735ff3193b8d3b1f9641dd
1 file changed