Add a new heuristic 'length64' to detect interior pointers
pointing at offset 64bit of a block, when the first 8 bytes contains
the block size - 8. This is e.g. used by sqlite3MemMalloc.

Patch by Matthias Schwarzott (with small modif)



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14179 a5019735-40e9-0310-863c-91ae7b9d1cf9