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)