Initialize Heap's collector pointers to null.

Test: Run ART test suite on host.
Bug: 30807718
Change-Id: I9b83b44df250b00af48dc1aa653d78aea84d0874
diff --git a/runtime/gc/heap.cc b/runtime/gc/heap.cc
index 88fbf78..b574c3b 100644
--- a/runtime/gc/heap.cc
+++ b/runtime/gc/heap.cc
@@ -230,6 +230,9 @@
       total_wait_time_(0),
       verify_object_mode_(kVerifyObjectModeDisabled),
       disable_moving_gc_count_(0),
+      semi_space_collector_(nullptr),
+      mark_compact_collector_(nullptr),
+      concurrent_copying_collector_(nullptr),
       is_running_on_memory_tool_(Runtime::Current()->IsRunningOnMemoryTool()),
       use_tlab_(use_tlab),
       main_space_backup_(nullptr),