Compile error with Xcode 6.3.1

[Renze Torensma]

I tried compiling Graal today with the jdk1.8.0_45 on OSX 10.10.3 with Xcode 6.3.1 installed, and got some compiler errors:

*/filebuff.cpp:109:8: error: 'this' pointer cannot be null in well-defined C++ code; pointer may be assumed to always convert to true [-Werror,-Wundefined-bool-conversion]

  if( !this ) return NULL;      // The empty buffer region

      ~^~~~

*/filebuff.cpp:122:8: error: 'this' pointer cannot be null in well-defined C++ code; pointer may be assumed to always convert to true [-Werror,-Wundefined-bool-conversion]

  if( !this ) return br;        // Merging into nothing

      ~^~~~

*/filebuff.cpp:244:8: error: 'this' pointer cannot be null in well-defined C++ code; pointer may be assumed to always convert to true [-Werror,-Wundefined-bool-conversion]

  if( !this ) return;           // Nothing to print

      ~^~~~

3 errors generated.

*/formssel.cpp:3393:8: error: 'this' pointer cannot be null in well-defined C++ code; pointer may be assumed to always convert to true [-Werror,-Wundefined-bool-conversion]

  if( !this ) return;

      ~^~~~

1 error generated.

It seems like this error is introduced by clang 6.1 which is bundled with Xcode 6.3: https://bugs.openjdk.java.net/browse/JDK-8077364?page=com.atlassian.streams.streams-jira-plugin:activity-stream-issue-tab