[key-manager] add Pskc::Clear() method (#4315)

src/core/thread/key_manager.cpp

@@ -84,7 +84,7 @@ KeyManager::KeyManager(Instance &aInstance)
     , mIsPskcSet(false)
 {
     mMasterKey = static_cast<const MasterKey &>(kDefaultMasterKey);
-    memset(&mPskc, 0, sizeof(mPskc));
+    mPskc.Clear();
     ComputeKey(mKeySequence, mKey);
 }
 

src/core/thread/key_manager.hpp

@@ -97,6 +97,12 @@ OT_TOOL_PACKED_BEGIN
 class Pskc : public otPskc
 {
 public:
+    /**
+     * This method clears the PSKc (sets all bytes to zero).
+     *
+     */
+    void Clear(void) { memset(this, 0, sizeof(*this)); }
+
     /**
      * This method evaluates whether or not the Thread PSKc values match.
      *