Remove properties HT from nested GC data

The properties HT may be a GC root itself, so we need to remove it.
I'm not sure this issue actually applies to PHP 7.2, but committing
it there to be safe. As seen from the test case, the handling here
is rather buggy on 7.2.

Author: nikic

Zend/tests/gc_042.phpt

@@ -0,0 +1,26 @@
+--TEST--
+Object properties HT may need to be removed from nested data
+--FILE--
+<?php
+
+class Test {
+    public function __destruct() {
+        $GLOBALS['x'] = $this;
+    }
+}
+
+$t = new Test;
+$t->x = new stdClass;
+$t->x->t = $t;
+$a = (array) $t->x;
+unset($t, $a);
+gc_collect_cycles();
+var_dump($x);
+
+// TODO: The destructor *should* be running here, but doesn't.
+// This works in PHP >= 7.3 though.
+
+?>
+--EXPECTF--
+Notice: Undefined variable: x in %s on line %d
+NULL

Zend/zend_gc.c

@@ -1007,6 +1007,10 @@ static void gc_remove_nested_data_from_buffer(zend_refcounted *ref, gc_root_buff
 					ref = Z_COUNTED_P(zv);
 					goto tail_call;
 				}
+				if (GC_ADDRESS(GC_INFO(ht)) != 0 && GC_REF_GET_COLOR(ht) == GC_BLACK) {
+					GC_TRACE_REF(ht, "removing from buffer");
+					GC_REMOVE_FROM_BUFFER(ht);
+				}
 			} else {
 				return;
 			}