spapr: fix memory leak in spapr_core_pre_plug()

In case of error, we must ensure the dynamically allocated base_core_type
is freed, like it is done everywhere else in this function.

This is a regression introduced in QEMU 2.9 by commit 8149e2992f78.

Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
