snmp plugin: Fix double free of request PDU

snmp_sess_synch_response() always frees request PDU, in both case of request
error and success. If error condition occurs inside of `while (status == 0)`
loop, double free of `req` happens.

Issue: #2291
Signed-off-by: Florian Forster <octo@collectd.org>
static void call_snmp_init_once(void) {
  static int have_init = 0;

  if (have_init == 0)
    init_snmp(PACKAGE_NAME);
  have_init = 1;
} /* void call_snmp_init_once */

static void call_snmp_init_once(void) {
  static int have_init = 0;

  if (have_init == 0)
    init_snmp(PACKAGE_NAME);
  have_init = 1;
} /* void call_snmp_init_once */
