[PARISC] fix null ptr deref in unwind.c

commit ffb45122766db220d0bf3d01848d575fbbcb6430 removed one too many args.
kallsyms_lookup is not safe to call with a NULL *modname. Paper bag over the
problem for the time being.

Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>
1 file changed