Bugfix: call cudaFree before resize of gpuVectors_

How could this have been working until now? We should not call cudaFree on uninitialized memory.

We have to free the old pointers before resize. This also prevents memory leakage in case the vector gets shrinked.

Apparently the previous code relied on the fact that resize somehow initialized the memory to null pointers or that cudaFree just ignores pointers it didn't allocate.