opencl: check if gpus.kernelname has to be free()'d before setting.

In this case it doesn't, since the function is only used for config
parsing (I hope). Add the check to prevent a memory leak, just in
case this changes in the future.

TODO: Option parsing functions should really be moved elsewhere and
renamed appropriately, like set_opt_kernel() or config_set_kernel(),
if config parsing is to remain as-is. It will probably change
drastically, so leaving as-is for now.