This patch adds a condition before overwriting exception_index fiels.
It is needed when exception_index is already set to some meaningful value.

Signed-off-by: Pavel Dovgalyuk <pavel.dovgaluk@ispras.ru>

Message-Id: <20171114081812.27640.26372.stgit@pasha-VirtualBox>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>