At least with certain versions of Boost and Clang, and when NULL is defined as
nullptr, compilation failed here.  To me, looks like an error of either Boost or
Clang; replacing "NULL" with "0" appears to be a good enough workaround.
(Identified by Jung-uk Kim.)

Change-Id: I702587abfabad6dba55a2b4c65a53e332b4ab048