Fix a possible endless loop situation in serf_linebuf_fetch as reported in r1814.
Fix a possible endless loop situation in serf_linebuf_fetch as reported in
r1814.

* buckets/buckets.c
  (serf_linebuf_fetch): Exit from the loop if the stream bucket did not have
       more data to read.

jandre/serf,jandre/serf,jandre/serf,jandre/serf