i386: kvmvapic: initialise imm32 variable

When processing Task Priorty Register(TPR) access, it could leak
automatic stack variable 'imm32' in patch_instruction().
Initialise the variable to avoid it.

Reported by: Donghai Zdh <donghai.zdh@alibaba-inc.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Message-Id: <1460013608-16670-1-git-send-email-ppandit@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>