add cudaGetLastError() in mallocBest

[wei-tan]

Such at the code can return the last error that has been produced by any of the runtime calls in the same host thread and resets it to cudaSuccess.
https://www.cs.cmu.edu/afs/cs/academic/class/15668-s11/www/cuda-doc/html/group__CUDART__ERROR_g0a933755a73d874d0d7051a3eb2aa533.html

If not, very subsequent cudaCheckLastError() will get a failure message.

`cudaError_t mallocBest ( void **devPtr, size_t size ) {

cudaError_t returnVal = cudaSuccess;

if (cudaMalloc(devPtr, size) != cudaSuccess) {

    cudaGetLastError();

}

}`