[tlv] change GetSize() return type to uint16_t to avoid overflow (#2870)

Credit to OSS-Fuzz.

src/core/common/tlvs.hpp

@@ -104,7 +104,7 @@ public:
      * @returns The total size include Type, Length, and Value fields.
      *
      */
-    uint8_t GetSize(void) const { return sizeof(Tlv) + mLength; }
+    uint16_t GetSize(void) const { return sizeof(Tlv) + mLength; }
 
     /**
      * This method returns a pointer to the Value.

src/core/meshcop/meshcop_tlvs.hpp

@@ -1336,7 +1336,7 @@ public:
      * @returns The total size of this entry (number of bytes).
      *
      */
-    uint8_t GetSize(void) const { return sizeof(ChannelMaskEntry) + mMaskLength; }
+    uint16_t GetSize(void) const { return sizeof(ChannelMaskEntry) + mMaskLength; }
 
     /**
      * This method clears the bit corresponding to @p aChannel in ChannelMask.