arch: tile: fix null pointer dereference on pt_regs pointer

arch: tile: fix null pointer dereference on pt_regs pointer

Cppcheck reports the following issue:
[arch/tile/kernel/stack.c:116]: (error) Possible null
  pointer dereference: p

In this case, on reporting on an odd fault, p is set to NULL
and immediately afterwords p is dereferenced iff
!kbt->profile is false.  Rather than doing this check just
return NULL rather than falling through to the potential
null pointer dereference (since the original intentional
outcome would be to return NULL anyhow) for this odd fault
case.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com> [tweaked lightly]