The next patch will start accessing the excp_vectors array earlier in
the function, so add a bounds check as first thing here.

This converts the empty return on POWERPC_EXCP_NONE to an error. This
exception number never reaches this function and if it does it
probably means something else went wrong up the line.

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20211229165751.3774248-3-farosas@linux.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>