Initialized initrd_ctx so we don't free a random pointer from the stack.

Currently, if "linux" fails, the "goto fail;" in grub_cmd_initrd sends us
into grub_initrd_close() without grub_initrd_init() being called, and thus
it never clears initrd_ctx->components.  grub_initrd_close() then frees that
address, which is stale data from the stack.  If the stack happens to have a
stale *address* there that matches a recent allocation, then you'll get a
double free later.

So initialize the memory up front.

Signed-off-by: Peter Jones <pjones@redhat.com>