transform algorithm with constant iterator

My tests revealed that when using constant iterator tupled and zipped gives segfault.
Here is the code which shows that:

``` c++
#include
#include
#include
#include
#include
#include
#include
#include

int main (int argc, const char* argv[])
{
namespace compute = boost::compute;

try {
int size = 128;
compute::vector xVector (size, 1.f);
compute::vector yVector (size, 2.f);
compute::vector zVector (size, 3.f);

compute::vector rVector (size, 0.f);

BOOST_COMPUTE_FUNCTION(float, function_tuple_3, (boost::tuple t),
{
return boost_tuple_get(t, 0);
});
BOOST_COMPUTE_FUNCTION(float, function_tuple_2, (boost::tuple t),
{
return boost_tuple_get(t, 0);
});

/*This was my original code where I found the bug*/
/*
compute::transform(
//first
compute::make_zip_iterator(
boost::make_tuple(
xVector.begin(),
yVector.begin(),
//zVector.begin() //when using zVector instead of constant_iterator it is working correctly.
compute::make_constant_iterator(4.f)
)
),
//last
compute::make_zip_iterator(
boost::make_tuple(
xVector.end(),
yVector.end(),
//zVector.end() // change it here accordingly
compute::make_constant_iterator(4.f, size)
)
),

rVector.begin(),
//function
function_tuple_3
);
*/

/* when not zipping constant iterator it seems fine */
/*
compute::transform(compute::make_constant_iterator(4.f),
compute::make_constant_iterator(4.f, size),
rVector.begin(),
compute::identity()
);
*/

/* when start to zip it, program crashes */
compute::transform(
compute::make_zip_iterator(
boost::make_tuple(
compute::make_constant_iterator(4.f),
compute::make_constant_iterator(4.f)
)
),
compute::make_zip_iterator(
boost::make_tuple(
compute::make_constant_iterator(4.f, size),
compute::make_constant_iterator(4.f, size)
)
),
rVector.begin(),
function_tuple_2
);
}
catch (compute::opencl_error &err)
{
std::cerr << err.what() << std::endl;
return -1;
}
catch (...)
{
std::cerr << "Boost compute failed with transforn operation" << std::endl;
return -2;
}
return 0;
}

```
##