parisc: Fix uninitialized variable usage in unwind.c

As noticed by Dave Anglin, the last commit introduced a small bug where
the potentially uninitialized r struct is used instead of the regs
pointer as input for unwind_frame_init(). Fix it.

Signed-off-by: Helge Deller <deller@gmx.de>
Reported-by: John David Anglin <dave.anglin@bell.net>