lib-storage: Fix bodystructure parsing crash if header is parsed twice

The second parsing will recreate the parser_ctx, discarding the old parsed
message_part.data for the header. On the second parsing
save_bodystructure_header=FALSE so the message_part.data isn't filled for
the header. Later on the bodystructure parsing assumes the data is set,
and crashes.

This only happened with mail_attachment_detection_options=add-flags-on-save
and Sieve script that first accessed a non-cached header and then used the
"body" extension.

Fixes segfault and also:
Panic: file imap-bodystructure.c: line 116 (part_write_body_multipart): assertion failed: (part->data != NULL)

src/lib-storage/index/index-mail-headers.c

@@ -313,7 +313,7 @@ void index_mail_parse_header(struct message_part *part,
 		if (data->save_bodystructure_header) {
 			i_assert(!data->save_bodystructure_body ||
 				 data->parser_ctx != NULL);
-			data->save_bodystructure_header = FALSE;
+			data->parsed_bodystructure_header = TRUE;
 		}
 		return;
 	}
@@ -414,6 +414,12 @@ static void index_mail_init_parser(struct index_mail *mail)
 			index_mail_set_message_parts_corrupted(&mail->mail.mail, error);
 			data->parts = NULL;
 		}
+		if (data->parts == NULL) {
+			/* The previous parsing didn't finish, so we're
+			   re-parsing the header. The new parts don't have data
+			   filled anymore. */
+			data->parsed_bodystructure_header = FALSE;
+		}
 	}
 
 	if (data->parts == NULL) {

src/lib-storage/index/index-mail.c

@@ -1193,7 +1193,7 @@ static int index_mail_parse_body(struct index_mail *mail,
 	if (data->save_bodystructure_body) {
 		/* bodystructure header is parsed, we want the body's mime
 		   headers too */
-		i_assert(!data->save_bodystructure_header);
+		i_assert(data->parsed_bodystructure_header);
 		message_parser_parse_body(data->parser_ctx,
 					  parse_bodystructure_part_header,
 					  mail->mail.data_pool);
@@ -1346,7 +1346,8 @@ static int index_mail_parse_bodystructure(struct index_mail *mail,
 		   a string */
 		index_mail_body_parsed_cache_bodystructure(mail, field);
 	} else {
-		if (data->save_bodystructure_header ||
+		if ((data->save_bodystructure_header &&
+		     !data->parsed_bodystructure_header) ||
 		    !data->save_bodystructure_body ||
 		    field == MAIL_CACHE_BODY_SNIPPET) {
 			/* we haven't parsed the header yet */

src/lib-storage/index/index-mail.h

@@ -118,6 +118,7 @@ struct index_mail_data {
 	bool save_body_snippet:1;
 	bool stream_has_only_header:1;
 	bool parsed_bodystructure:1;
+	bool parsed_bodystructure_header:1;
 	bool hdr_size_set:1;
 	bool body_size_set:1;
 	bool messageparts_saved_to_cache:1;