hw/ide/core.c: Prevent SIGSEGV during migration

hw/ide/core.c: Prevent SIGSEGV during migration

The other callers to blk_set_enable_write_cache() in this file
already check for s->blk == NULL.

Signed-off-by: Don Slutz <dslutz@verizon.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1416259239-13281-1-git-send-email-dslutz@verizon.com
Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
(cherry picked from commit 6b896ab261942f441a16836e3fa3c83f3f4488b9)

Conflicts:
	hw/ide/core.c

*removed dependency on 4be746345

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>