pstore: Fix NULL pointer fault if get NULL prz in ramoops_get_next_prz
ramoops_get_next_prz get the prz according the paramters. If it get a
uninitialized prz, access its members by following persistent_ram_old_size(prz)
will cause a NULL pointer crash.
Ex: if ftrace_size is 0, fprz will be NULL.
Fix it by return NULL in advance.
Signed-off-by: Liu ShuoX <shuox.liu@intel.com> Acked-by: Kees Cook <keescook@chromium.org> Signed-off-by: Tony Luck <tony.luck@intel.com>