PARQUET-232: minor compilation issue

I find out some very minor issue when I tried to compile the reader on my environment due to some namespace clashing.
As example shared_ptr and unordered_map are also in C++11 std namespace. Some compile don't like it.
Also I find that with my test files that I'm reading there was a dereference to a null pointer, if the field is required definition_level_decoder_ is null.

Author: Fabrizio Fabbri <ffabbri4@bloomberg.net>

Closes #7 from ffabbri4/candidate and squashes the following commits:

3f8a445 [Fabrizio Fabbri] Fix null pointer when the repetition level is required.
2948584 [Fabrizio Fabbri] Explicit include boost shared pointer header. specify boost::unordered_map as on C++11 is ambiguous.

Change-Id: Iec1d78d3e199eac8cc0cfe68324a9bc219d4802e

Author: ffabbri4

cpp/src/parquet/parquet.h

@@ -19,6 +19,7 @@
 #include <sstream>
 #include <boost/cstdint.hpp>
 #include <boost/scoped_ptr.hpp>
+#include <boost/shared_ptr.hpp>
 #include <boost/unordered_map.hpp>
 #include "gen-cpp/parquet_constants.h"
 #include "gen-cpp/parquet_types.h"
@@ -212,7 +213,9 @@ inline ByteArray ColumnReader::GetByteArray(int* def_level, int* rep_level) {
 
 inline bool ColumnReader::ReadDefinitionRepetitionLevels(int* def_level, int* rep_level) {
   *rep_level = 1;
-  if (!definition_level_decoder_->Get(def_level)) ParquetException::EofException();
+  if (definition_level_decoder_ && !definition_level_decoder_->Get(def_level)) {
+    ParquetException::EofException();
+  }
   --num_buffered_values_;
   return *def_level == 0;
 }