using pattern mode to detect use after free before reuse

_From [zhao...@google.com](https://code.google.com/u/106321947286816917100/) on February 14, 2012 16:45:42_

in pattern mode, the actually freed memory is neither in malloc rbtree nor in delayed free rbtree, in which case we cannot detect.
we can:
1. increase delayed free size
2. maintain the information in pattern malloc rbtree
3. check if the memory is in heap region.

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=786_