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.

upstream-commit-id: 6b896ab261942f441a16836e3fa3c83f3f4488b9

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
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Release-Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Conflicts:
	hw/ide/core.c