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

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)