RRM: Fix wpas_rrm_send_msr_report() loop handling

RRM: Fix wpas_rrm_send_msr_report() loop handling

The while (len) loop was updating the next pointer at the end even when
len == 0, i.e., when the new next value won't be used. This could result
in reading one octet beyond the end of the allocated response wpabuf.
While the read value is not really used in practice, this is not correct
behavior, so fix this by skipping the unnecessary next pointer update in
len == 0 case.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>