Fixed a serious problem with IMAP header fetch. memreadline() returns a pointer...

Fixed a serious problem with IMAP header fetch.  memreadline() returns a pointer to the *next* line, or null if we're at the end.  In the IMAP header parsing routine we were checking for null at the top of a while loop, where we should have been checking for null at the end of a do-while loop.  As a result, the last header line was being discarded under any circumstances.