Blackfin: do not try displaying the end of the stack

The end of the stack may not be valid (and that could be OK), so do not
attempt to parse it.  If we do, we might use a bad pointer in kernel space
which makes things panic().

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>