ART: Replace NULL to nullptr in the optimizing compiler

Replace macro NULL to the nullptr variation for C++.

Change-Id: Ib6e48dd4bb3c254343383011b67372622578ca76
Signed-off-by: Jean Christophe Beyler <jean.christophe.beyler@intel.com>
diff --git a/compiler/optimizing/code_generator.cc b/compiler/optimizing/code_generator.cc
index 6f424ce..5f7a121 100644
--- a/compiler/optimizing/code_generator.cc
+++ b/compiler/optimizing/code_generator.cc
@@ -369,7 +369,7 @@
     uint32_t native_offset = pc_info.native_pc;
     uint32_t dex_pc = pc_info.dex_pc;
     const uint8_t* references = dex_gc_map.FindBitMap(dex_pc, false);
-    CHECK(references != NULL) << "Missing ref for dex pc 0x" << std::hex << dex_pc;
+    CHECK(references != nullptr) << "Missing ref for dex pc 0x" << std::hex << dex_pc;
     builder.AddEntry(native_offset, references);
   }
 }