[ip6] remove default constructor for Ip6::Option (#8712)

All sub-classes of `Option` provide `Init()` method to initialize
the Option.

This commit also updates `Lowpan::CompressExtensionHeader()` to
check that we did read an option from message before checking if
it can be compressed.

src/core/net/ip6_headers.cpp

@@ -38,6 +38,9 @@
 namespace ot {
 namespace Ip6 {
 
+//---------------------------------------------------------------------------------------------------------------------
+// Header
+
 Error Header::ParseFrom(const Message &aMessage)
 {
     Error error = kErrorParse;
@@ -63,5 +66,15 @@ bool Header::IsValid(void) const
     return IsVersion6() && ((sizeof(Header) + GetPayloadLength()) <= kMaxLength);
 }
 
+//---------------------------------------------------------------------------------------------------------------------
+// PadNOption
+
+void PadNOption::Init(uint8_t aPadLength)
+{
+    SetType(kType);
+    SetLength(aPadLength - sizeof(Option));
+    memset(mPad, kData, aPadLength - sizeof(Option));
+}
+
 } // namespace Ip6
 } // namespace ot

src/core/net/ip6_headers.hpp

@@ -450,16 +450,6 @@ OT_TOOL_PACKED_BEGIN
 class Option
 {
 public:
-    /**
-     * Default constructor.
-     *
-     */
-    Option(void)
-        : mType(0)
-        , mLength(0)
-    {
-    }
-
     /**
      * This method returns the IPv6 Option Type value.
      *
@@ -545,12 +535,7 @@ public:
      * @param[in]  aPadLength  The length of needed padding. Allowed value from range 2-7.
      *
      */
-    void Init(uint8_t aPadLength)
-    {
-        SetType(kType);
-        SetLength(aPadLength - sizeof(Option));
-        memset(mPad, kData, aPadLength - sizeof(Option));
-    }
+    void Init(uint8_t aPadLength);
 
 private:
     uint8_t mPad[kMaxLength];

src/core/thread/lowpan.cpp

@@ -472,7 +472,8 @@ Error Lowpan::CompressExtensionHeader(Message &aMessage, FrameBuilder &aFrameBui
     // Pad1 or PadN option MAY be elided by the compressor."
     if (aNextHeader == Ip6::kProtoHopOpts || aNextHeader == Ip6::kProtoDstOpts)
     {
-        uint16_t    offset = aMessage.GetOffset();
+        uint16_t    offset    = aMessage.GetOffset();
+        bool        hasOption = false;
         Ip6::Option option;
 
         while ((offset - aMessage.GetOffset()) < len)
@@ -487,19 +488,24 @@ Error Lowpan::CompressExtensionHeader(Message &aMessage, FrameBuilder &aFrameBui
             {
                 offset += option.GetSize();
             }
+
+            hasOption = true;
         }
 
-        // Check if the last option can be compressed.
-        if (option.GetType() == Ip6::Pad1Option::kType)
+        if (hasOption)
         {
-            padLength = sizeof(Ip6::Pad1Option);
-        }
-        else if (option.GetType() == Ip6::PadNOption::kType)
-        {
-            padLength = option.GetSize();
-        }
+            // Check if the last option can be compressed.
+            if (option.GetType() == Ip6::Pad1Option::kType)
+            {
+                padLength = sizeof(Ip6::Pad1Option);
+            }
+            else if (option.GetType() == Ip6::PadNOption::kType)
+            {
+                padLength = option.GetSize();
+            }
 
-        len -= padLength;
+            len -= padLength;
+        }
     }
 
     VerifyOrExit(aMessage.GetOffset() + len + padLength <= aMessage.GetLength(), error = kErrorParse);