fix bug in -delay_frees_stack -replace_malloc: set user_data to NULL after freeing it
fix bug in -delay_frees_stack -replace_malloc: set user_data to NULL after
freeing it

git-svn-id: fe3cd4388a6e5a968b09397a3d721fe124e8117a@928 5b54908e-ead4-fc18-b48a-0285e6dbc711

rnk/drmemory,rnk/drmemory,rnk/drmemory,rnk/drmemory