Fix CompilationUnit constructor issues.

Ensure target64 is initialized. Switch from NULL to nullptr as the former gives
compilation errors with std::unique_ptr from libc++.

Change-Id: I7153368d9324d10ef257f7c7ce9571a1753e5ba8
diff --git a/compiler/dex/frontend.cc b/compiler/dex/frontend.cc
index d9d392f..a224de2 100644
--- a/compiler/dex/frontend.cc
+++ b/compiler/dex/frontend.cc
@@ -78,31 +78,32 @@
   0;
 
 CompilationUnit::CompilationUnit(ArenaPool* pool)
-  : compiler_driver(NULL),
-    class_linker(NULL),
-    dex_file(NULL),
-    class_loader(NULL),
+  : compiler_driver(nullptr),
+    class_linker(nullptr),
+    dex_file(nullptr),
+    class_loader(nullptr),
     class_def_idx(0),
     method_idx(0),
-    code_item(NULL),
+    code_item(nullptr),
     access_flags(0),
     invoke_type(kDirect),
-    shorty(NULL),
+    shorty(nullptr),
     disable_opt(0),
     enable_debug(0),
     verbose(false),
-    compiler(NULL),
+    compiler(nullptr),
     instruction_set(kNone),
+    target64(false),
     num_dalvik_registers(0),
-    insns(NULL),
+    insns(nullptr),
     num_ins(0),
     num_outs(0),
     num_regs(0),
     compiler_flip_match(false),
     arena(pool),
     arena_stack(pool),
-    mir_graph(NULL),
-    cg(NULL),
+    mir_graph(nullptr),
+    cg(nullptr),
     timings("QuickCompiler", true, false) {
 }