m32r: hole in shifting pc back

It's a userland pointer; worse, an untrustable one since ptrace
has just provided a chance to modify it.

X-Roothole-Covering-Cabal: TINRCC
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>