xfs: avoid null *src in memcpy call in xlog_write

The gcc undefined behavior sanitizer caught this; surely
any sane memcpy implementation will no-op if size == 0,
but behavior with a *src of NULL is technically undefined
(declared nonnull), so avoid it here.

We are actually in this situation frequently via
xlog_commit_record(), because:

        struct xfs_log_iovec reg = {
                .i_addr = NULL,
                .i_len = 0,
                .i_type = XLOG_REG_TYPE_COMMIT,
        };
Reported-by: NEric Sandeen <sandeen@redhat.com>
Signed-off-by: NDave Chinner <dchinner@redhat.com>
Reviewed-by: NEric Sandeen <sandeen@redhat.com>
Signed-off-by: NDave Chinner <david@fromorbit.com>

fs/xfs/xfs_log.c

@@ -2422,11 +2422,20 @@ xlog_write(
 						     &partial_copy_len);
 			xlog_verify_dest_ptr(log, ptr);
 
-			/* copy region */
+			/*
+			 * Copy region.
+			 *
+			 * Unmount records just log an opheader, so can have
+			 * empty payloads with no data region to copy. Hence we
+			 * only copy the payload if the vector says it has data
+			 * to copy.
+			 */
 			ASSERT(copy_len >= 0);
-			memcpy(ptr, reg->i_addr + copy_off, copy_len);
-			xlog_write_adv_cnt(&ptr, &len, &log_offset, copy_len);
-
+			if (copy_len > 0) {
+				memcpy(ptr, reg->i_addr + copy_off, copy_len);
+				xlog_write_adv_cnt(&ptr, &len, &log_offset,
+						   copy_len);
+			}
 			copy_len += start_rec_copy + sizeof(xlog_op_header_t);
 			record_cnt++;
 			data_cnt += contwr ? copy_len : 0;