reworked talloc() to in order to help track down invalid

reworked talloc() to in order to help track down invalid
memory access.

Tridge, I don't think using 0xdeadbeef for size allocations
of 0 is going to work.  I ended up having to use NULL
as much code that works on UNISTR checks to see if the buffer ptr
is NULL.  So valid code ends up with a seg fault.

Rather than rewriting it all, I added a DEBUG_TALLOC
#ifdef in talloc.h that sets a macro BAD_PTR.
This is the value assigned to ptr for an allocation of 0 bytes.

jerry