[heapstat] callstack improvements: ignore bogus frames, better walking

_From [derek.br...@gmail.com](https://code.google.com/u/117968039472581148324/) on December 10, 2010 17:58:00_

PR 469043

a few issues/improvement suggestions:

1) should these be identical and we should ignore the bogus frames?
CALLSTACK 2
0x1392c8f8 0x145b155b 0x00000015 0x00000840 error end
CALLSTACK 3
0x1392c8f8 0x145b155b 0x00000023 0x00000840 error end
CALLSTACK 4
0x1392c8f8 0x145b155b 0x00000029 0x00000840 error end

2) can we improve our callstack walking? should take a look and see
whether we can do better than the bogus frames we often hit:
CALLSTACK 36809
0x1392c8f8 0x1389d7d9 0xffc86d50 0x00010101 error end

3) the allocated-at-startup mallocs have the 1st callstack which is empty:
CALLSTACK 1
error end
should perhaps identify in some other way to distinguish from later calls
that just have bad callstacks? though don't expect completely empty
callstacks at post-malloc point later on.

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=146_