bpo-33803: Fix a crash in hamt.c (#7504)

Author: 1st1

Misc/NEWS.d/next/Core and Builtins/2018-06-07-20-18-38.bpo-33803.n-Nq6_.rst

@@ -0,0 +1,2 @@
+Fix a crash in hamt.c caused by enabling GC tracking for an object that
+hadn't all of its fields set to NULL.

Python/hamt.c

@@ -2476,6 +2476,8 @@ hamt_alloc(void)
     if (o == NULL) {
         return NULL;
     }
+    o->h_count = 0;
+    o->h_root = NULL;
     o->h_weakreflist = NULL;
     PyObject_GC_Track(o);
     return o;