pattern mode makes the size of freed string non-determinstic

_From [zhao...@google.com](https://code.google.com/u/106321947286816917100/) on February 27, 2012 16:26:00_

In unix_syscalls test case, in unaddr_open,
a buffer of string is freed and DrMemory fills the buffer with pattern value, later when that buffer is used as a parameter in system call, DrMemory cannot determine the correct size.

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=804_