Codechange: All ParagraphLayoutFactory::AppendToBuffer assume that the buffer has at least some space.

Assert on that.

src/os/macosx/string_osx.h

@@ -63,6 +63,7 @@ public:
 	 */
 	static size_t AppendToBuffer(CharType *buff, const CharType *buffer_last, char32_t c)
 	{
+		assert(buff < buffer_last);
 		if (c >= 0x010000U) {
 			/* Character is encoded using surrogates in UTF-16. */
 			if (buff + 1 <= buffer_last) {