hw/omap_intc.c: Avoid crash on access to nonexistent banked registers

Avoid a crash due to null pointer dereference if a guest attempts
to access banked registers for a nonexistent bank. Spotted by
Coverity (see bug 887883).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
