[csl] ignore zero valued CSL IE period (#9414)

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() >= kMinCslIePeriod);
+
     child->SetCslPeriod(csl->GetPeriod());
     child->SetCslPhase(csl->GetPhase());
     child->SetCslSynchronized(true);

src/core/mac/mac.hpp

@@ -73,7 +73,7 @@ namespace Mac {
 
 constexpr uint32_t kDataPollTimeout =
     OPENTHREAD_CONFIG_MAC_DATA_POLL_TIMEOUT; ///< Timeout for receiving Data Frame (in msec).
-constexpr uint32_t kSleepDelay = 300;        ///< Max sleep delay when frame is pending (in msec)
+constexpr uint32_t kSleepDelay = 300;        ///< Max sleep delay when frame is pending (in msec).
 
 constexpr uint16_t kScanDurationDefault = OPENTHREAD_CONFIG_MAC_SCAN_DURATION; ///< Duration per channel (in msec).
 
@@ -87,6 +87,8 @@ constexpr uint8_t kMaxFrameRetriesCsl             = 0;
 
 constexpr uint8_t kTxNumBcast = OPENTHREAD_CONFIG_MAC_TX_NUM_BCAST; ///< Num of times broadcast frame is tx.
 
+constexpr uint16_t kMinCslIePeriod = OPENTHREAD_CONFIG_MAC_CSL_MIN_PERIOD;
+
 /**
  * Defines the function pointer called on receiving an IEEE 802.15.4 Beacon during an Active Scan.
  *