Added handling for malloc/realloc returning null

Added handling for malloc/realloc returning null. The null value is now
returned from the wrapper function and a node is not inserted.
Added DMT_ABORTNULL macro check

Defining the macro DMT_ABORTNULL will make _dmt_alloc() and
_dmt_realloc() abort with an error message if they receive a null
pointer from malloc() or realloc(). If this macro is not set they return
a null pointer instead of aborting.