oclMat sub matrix function fix 

I found when use the colRange or rowRange to get sub matrix the program will be crash.
 I begin to debug it, and then I found the context of the opencl device not be initialized in function "inline oclMat::oclMat(const oclMat &m, const Range &rRange, const Range &cRange)" of the file "matrix_operations.hpp".
I only add 1 line code "clCxt = m.clCxt;" then compile it and then the crash problem is fixed.