pstore: handle zero-sized prz in series

ramoops_pstore_read fails to return the next in a prz
series after first zero-sized entry, not venturing to
the next non-zero entry.

Signed-off-by: Mark Salyzyn <salyzyn@android.com>
Change-Id: Iedce3b94c13917da33be44e1d80811757774c793