pstore: skip zero size persistent ram buffer in traverse

In ramoops_pstore_read, a valid prz pointer with zero size buffer will
break traverse of all persistent ram buffers.  The latter buffer might be
lost.
Signed-off-by: Liu ShuoX <shuox.liu@intel.com>
Cc: "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>
Cc: Colin Cross <ccross@android.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>

fs/pstore/ram.c

@@ -120,12 +120,12 @@ ramoops_get_next_prz(struct persistent_ram_zone *przs[], uint *c, uint max,
 
 	prz = przs[i];
 
-	if (update) {
-		/* Update old/shadowed buffer. */
+	/* Update old/shadowed buffer. */
+	if (update)
 		persistent_ram_save_old(prz);
-		if (!persistent_ram_old_size(prz))
-			return NULL;
-	}
+
+	if (!persistent_ram_old_size(prz))
+		return NULL;
 
 	*typep = type;
 	*id = i;