Fix chk_csetpointer() to deal with an s-file's cset file pointer being NULL.
Fix chk_csetpointer() to deal with an s-file's cset file pointer
being NULL.

bk: 3b72aa5aEr6UxSk0g3mk0nmbcIZxOA
bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper