unwindstack: fix dangling pointer in LocalUpdatableMaps.

Previously, when reparsing /proc/self/maps, we would remove duplicate
MapInfo entries, but leave the following entry's prev_map pointing
toward the soon-to-be-deleted MapInfo, leading to explosions.

Test: libunwindstack_test
Test: booted with libfdtrack.so preloaded
Change-Id: Ibfb7a8712540fe3aaadc10e9c31938f6ecddf17b