trace.c: ensure NULL is not passed to printf

trace.c: ensure NULL is not passed to printf

GNU printf, and many others, will print the string "(null)" if a NULL
pointer is passed as the argument to a "%s" format specifier.  Some
implementations (like on Solaris) do not detect a NULL pointer and will
produce a segfault in this case.

So, fix this by ensuring that pointer variables do not contain the value
NULL.  Assign the string "(null)" to the variables are NULL.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>