Fix C++ conversion compilation error in CentOS 7

[ 75%] Building CXX object c++/test/CMakeFiles/orc-test.dir/TestMurmur3.cc.o
/root/orc/c++/test/TestMurmur3.cc: In member function 'virtual void orc::TestMurmur3_testHash64_Test::TestBody()':
/root/orc/c++/test/TestMurmur3.cc:36:53: error: conversion to 'uint32_t {aka unsigned int}' from 'long unsigned int' may alter its value [-Werror=conversion]
     uint32_t len = sizeof(origin) / sizeof(uint8_t) - 1;
                                                     ^
cc1plus: all warnings being treated as errors
make[2]: *** [c++/test/CMakeFiles/orc-test.dir/TestMurmur3.cc.o] Error 1
make[1]: *** [c++/test/CMakeFiles/orc-test.dir/all] Error 2
make: *** [all] Error 2
FAILED centos7