lib-mail: start relying on mail_html2text_deinit(NULL) being a no-op

Cleanup performed with the following semantic patch:

	@@
	expression E;
	@@

	- if (E != NULL) {
	-       mail_html2text_deinit(&E);
	- }
	+ mail_html2text_deinit(&E);

src/lib-mail/message-snippet.c

@@ -138,8 +138,7 @@ int message_snippet_generate(struct istream *input,
 	i_assert(ret != 0);
 	message_decoder_deinit(&decoder);
 	message_parser_deinit(&parser, &parts);
-	if (ctx.html2text != NULL)
-		mail_html2text_deinit(&ctx.html2text);
+	mail_html2text_deinit(&ctx.html2text);
 	pool_unref(&pool);
 	return input->stream_errno == 0 ? 0 : -1;
 }