Switch NULL to C++11 nullptr in source/Interpreter

Patch by Robert Matusewicz

llvm-svn: 206711
diff --git a/lldb/source/Interpreter/OptionValueFormat.cpp b/lldb/source/Interpreter/OptionValueFormat.cpp
index 34d3672..296dd98 100644
--- a/lldb/source/Interpreter/OptionValueFormat.cpp
+++ b/lldb/source/Interpreter/OptionValueFormat.cpp
@@ -49,7 +49,7 @@
     case eVarSetOperationAssign:
         {
             Format new_format;
-            error = Args::StringToFormat (value_cstr, new_format, NULL);
+            error = Args::StringToFormat (value_cstr, new_format, nullptr);
             if (error.Success())
             {
                 m_value_was_set = true;