Fix gcc warnings

Fix #12675, close #12676

boostregex/AnsiDocumentIterator.h

@@ -78,6 +78,8 @@ public:
 		return charAt(m_pos);
 	}
 
+	AnsiDocumentIterator& operator = (const AnsiDocumentIterator& other) = default;
+
 	AnsiDocumentIterator& operator ++ ()
 	{
 		PLATFORM_ASSERT(m_pos < m_end);

boostregex/UTF8DocumentIterator.h

@@ -61,6 +61,8 @@ public:
 		return *this;
 	}
 
+	UTF8DocumentIterator& operator = (const UTF8DocumentIterator& other) = default;
+
 	UTF8DocumentIterator& operator ++ ();
 	UTF8DocumentIterator& operator -- ();
 
@@ -79,7 +81,7 @@ private:
 	}
 
 	Sci::Position m_pos = 0;
-	wchar_t m_character[2];
+	wchar_t m_character[2]{};
 	Sci::Position m_end = 0;
 	int m_characterIndex = 0;
 	int m_utf8Length = 0;