Ensure data is fully collected first in finalize

* src/mimedataprovider.cpp (MimeDataProvider::finalize): Flush
rawbuf before handling prot headers.

For very short messages text/plain messages that were not
fully collected this could otherwise lead to the case where
the content was not visible as it would be collected
into the ph_helpbuf after the code which checked for contents
of that buffer.

• GnuPG-Bug-Id: T6357