[csl] ignore zero valued CSL IE period

Ignore a CSL IE with period set to 0.

Accepting it would eventually lead to a division by 0 fault in
`CslTxScheduler::GetNextCslTransmissionDelay`.

src/core/mac/mac.cpp

@@ -2329,6 +2329,8 @@ void Mac::ProcessCsl(const RxFrame &aFrame, const Address &aSrcAddr)
     VerifyOrExit(child != nullptr);
 
     csl = reinterpret_cast<const CslIe *>(cur + sizeof(HeaderIe));
+    VerifyOrExit(csl->GetPeriod() != 0);
+
     child->SetCslPeriod(csl->GetPeriod());
     child->SetCslPhase(csl->GetPhase());
     child->SetCslSynchronized(true);