git for-each-ref crashes for %(body) with tags

Hi,

see gdb trace below. Unfortunately, I don't have a repo that I can readily share to demonstrate this.

(gdb) r for-each-ref --format '%(body)' refs/tags ..

Breakpoint 1, grab_sub_body_contents (val=<value optimized out>,
deref=0, obj=<value optimized out>, buf=0x8146ad0, sz=161)
    at builtin-for-each-ref.c:494
494                             v->s = xstrdup(bodypos);
(gdb) p bodypos
$10 = 0x0

(gdb) n 496 v->s = xstrdup(subpos); (gdb) n

Program received signal SIGSEGV, Segmentation fault. 0x4d9ff013 in strlen () from /lib/tls/i686/cmov/libc.so.6