On MinGW, GCC 4.7.2 complains about
operation on 'p->m[end]' may be undefined
Fix this by replacing the faulty lines with those of 69825ca from
https://github.com/ned14/nedmalloc/blob/master/nedmalloc.c
Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>