Add missing field init in constructors

Adds missing field initializations detected by Eclipse IDE.

Change-Id: I5f2f32bcccb12545fc9f0b42fcec74a23dc81376
diff --git a/runtime/hprof/hprof.cc b/runtime/hprof/hprof.cc
index a9a236f..ee6b020 100644
--- a/runtime/hprof/hprof.cc
+++ b/runtime/hprof/hprof.cc
@@ -421,6 +421,7 @@
         fd_(fd),
         direct_to_ddms_(direct_to_ddms),
         start_ns_(NanoTime()),
+        output_(nullptr),
         current_heap_(HPROF_HEAP_DEFAULT),
         objects_in_segment_(0),
         next_string_id_(0x400000),