Fix a C++11 error.

This merges patch by Nico Weber.

New clang versions complain in C++11 mode that:

error: cannot initialize a variable of type 'void *' with an rvalue of type 'uintptr_t' (aka 'unsigned long')

This same change was done for the google-internal version of tcmalloc too.

Reviewed-at: https://codereview.appspot.com/12132043



git-svn-id: http://gperftools.googlecode.com/svn/trunk@238 6b5cf1ce-ec42-a296-1ba9-69fdba395a50