pstore/ram: Do not treat empty buffers as valid

The ramoops backend currently calls persistent_ram_save_old() even
if a buffer is empty. While this appears to work, it is does not seem
like the right thing to do and could lead to future bugs so lets avoid
that. It also prevents misleading prints in the logs which claim the
buffer is valid.

I got something like:

	found existing buffer, size 0, start 0

When I was expecting:

	no valid data in buffer (sig = ...)

This bails out early (and reports with pr_debug()), since it's an
acceptable state.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Co-developed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>