HTTP/2: fix indirect reprioritization. Previously, streams that were indirectly reprioritized (either because of a new exclusive dependency on their parent or because of removal of their parent from the dependency tree), didn't have their pointer to the parent node updated. This broke detection of circular dependencies and, as a result, nginx worker would crash due to stack overflow whenever such dependency was introduced. Found with afl-fuzz. Signed-off-by: Piotr Sikora <piotrsikora@google.com>