KVM: s390: Fix memory leak on busy SIGP stop

commit 7dfc63cf
(KVM: s390: allow only one SIGP STOP (AND STORE STATUS) at a time)
introduced a memory leak if a sigp stop is already pending. Free
the allocated inti structure.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>