CDRIVER-735 segfault in aggregate

And update doc for mongoc_topology_select to warn that some pointer
parameters cannot be NULL.

Author: ajdavis

src/mongoc/mongoc-collection.c

@@ -236,12 +236,20 @@ mongoc_collection_aggregate (mongoc_collection_t       *collection, /* IN */
    bson_return_val_if_fail (collection, NULL);
    bson_return_val_if_fail (pipeline, NULL);
 
+   if (!read_prefs) {
+      read_prefs = collection->read_prefs;
+   }
+
    selected_server = mongoc_topology_select(collection->client->topology,
                                             MONGOC_SS_READ,
                                             read_prefs,
                                             15,
                                             &error);
 
+   if (!selected_server) {
+      return NULL;
+   }
+
    use_cursor = selected_server->max_wire_version >= 1;
 
    bson_init (&command);

src/mongoc/mongoc-topology.c

@@ -338,7 +338,7 @@ _mongoc_topology_do_blocking_scan (mongoc_topology_t *topology) {
 /*
  *-------------------------------------------------------------------------
  *
- * _mongoc_topology_select --
+ * mongoc_topology_select --
  *
  *       Selects a server description for an operation based on @optype
  *       and @read_prefs.
@@ -348,6 +348,15 @@ _mongoc_topology_do_blocking_scan (mongoc_topology_t *topology) {
  *
  *       NOTE: this method locks and unlocks @topology's mutex.
  *
+ * Parameters:
+ *       @topology: The topology.
+ *       @optype: Whether we are selecting for a read or write operation.
+ *       @read_prefs: Required, the read preferences for the command.
+ *       @local_threshold_ms: Maximum latency *beyond* the nearest server
+ *          among which to randomly select servers. See Server Selection
+ *          Spec.
+ *       @error: Required, out pointer for error info.
+ *
  * Returns:
  *       A mongoc_server_description_t, or NULL on failure, in which case
  *       @error will be set.