fetcher/soup: Fix gcc `-fanalyzer` warning

In general, we're probably going to need to change most of our
`g_return_if_fail` to `g_assert`.  The analyzer flags that
the function can return `NULL`, but the caller isn't prepared for
this.

In practice, let's abort.

src/libostree/ostree-fetcher-soup.c

@@ -183,7 +183,7 @@ static OstreeFetcherPendingURI *
 pending_uri_ref (OstreeFetcherPendingURI *pending)
 {
   gint refcount;
-  g_return_val_if_fail (pending != NULL, NULL);
+  g_assert (pending);
   refcount = g_atomic_int_add (&pending->ref_count, 1);
   g_assert (refcount > 0);
   return pending;