obstack.c: Fix some sparse warnings

obstack.c: Fix some sparse warnings

In particular, sparse issues the following warnings:

    compat/obstack.c:176:17: warning: Using plain integer as NULL pointer
    compat/obstack.c:224:17: warning: Using plain integer as NULL pointer
    compat/obstack.c:324:16: warning: Using plain integer as NULL pointer
    compat/obstack.c:329:16: warning: Using plain integer as NULL pointer
    compat/obstack.c:347:16: warning: Using plain integer as NULL pointer
    compat/obstack.c:362:19: warning: Using plain integer as NULL pointer
    compat/obstack.c:379:29: warning: Using plain integer as NULL pointer
    compat/obstack.c:399:1: error: symbol 'print_and_abort' redeclared with \
        different type (originally declared at compat/obstack.c:95) \
        - different modifiers

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>