fix short writes when not using SERVE-EVENT

fix short writes when not using SERVE-EVENT

  Regression since 1.0.42.43, lp#820599.

  Thanks to Robert Brown.

  If we remain in the loop, we need to update the local HEAD
  variable in the event of a short write.

  NOTE: No test case yet, as this is dependant on the size of kernel
  buffers, and trying to catch the short-write -case is tricky.