imap: Fixed calculation of transfer when partial FETCH received

The transfer size would be calculated incorrectly if the email contained
within the FETCH response, had been partially received by the pingpong
layer. As such the following, example output, would be seen if the
amount remaining was smaller than the amount received:

* Excess found in a non pipelined read: excess = 1394, size = 262,
  maxdownload = 262, bytecount = 1374
* transfer closed with -1112 bytes remaining to read

Bug: http://curl.haxx.se/mail/lib-2013-08/0170.html
Reported-by: John Dunn