pstore: don't OOPS when there are no ftrace zones

We'll OOPS in ramoops_get_next_prz() if the platform didn't ask for any
ftrace zones (i.e., cxt->fprzs will be NULL). Let's just skip this
entire FTRACE section if there's no 'fprzs'.

Regression seen on a coreboot/depthcharge-based Chromebook.

Fixes: 2fbea82b ("pstore: Merge per-CPU ftrace records into one")
Cc: Joel Fernandes <joelaf@google.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>