sm: pdpctx_timer_stop(): warn about timer not running

Similarly to 282de03, this commit turns errors like:

  DGPRS ERROR PDP(---/0) Stopping PDP timer 3395 but 0 is running

into warnings with a more accurate reason:

  DGPRS ERROR PDP(---/0) Stopping *inactive* PDP timer 3395

Change-Id: I64932d1cbd93667ab2e94e04691d6601435dba81
Related: 282de03 "gmm: mmctx_timer_stop(): warn about timer not running"

src/sgsn/gprs_sm.c

@@ -114,9 +114,14 @@ static void pdpctx_timer_start(struct sgsn_pdp_ctx *pdp, unsigned int T)
 
 static void pdpctx_timer_stop(struct sgsn_pdp_ctx *pdp, unsigned int T)
 {
-	if (pdp->T != T)
+	if (!osmo_timer_pending(&pdp->timer)) {
+		LOGPDPCTXP(LOGL_NOTICE, pdp, "Stopping *inactive* PDP timer %u\n", T);
+		return;
+	}
+	if (pdp->T != T) {
 		LOGPDPCTXP(LOGL_ERROR, pdp, "Stopping PDP timer %u but "
 			"%u is running\n", T, pdp->T);
+	}
 	osmo_timer_del(&pdp->timer);
 }