soup: Hold a ref to the pending URI during completion processing

soup: Hold a ref to the pending URI during completion processing

It was reported that in the range request handling, we called `remove_pending()`
twice (once in processing it, and once potentially in the local_error cleanup),
and this could be viewed as a use-after-free. However, right now the range
cleanup and `local_error` being set are mututally exclusive.
Further, the task object already holds a strong reference, so I observed the
refcount was 2. For both of these reasons, there is no use-after-free in
practice.

Reported-By: "Siddharth Sharma" <siddharth@redhat.com>
Closes: #774
Approved by: jlebon