setting --opencl-devices=0 fails with "Error CL_INVALID_BUFFER_SIZE when calling clCreateBuffer to create hash scratchpads buffer."
Running with "--opencl-platform=1 --opencl-devices=0" shows error
```
 * VERSIONS:     XMRig/2.6.1 libuv/1.19.2 OpenCL/2.0 MSVC/2017
 * CPU:          Intel(R) Core(TM) i5-7600K CPU @ 3.80GHz x64 AES-NI
 * ALGO:         cryptonight, donate=1%
 * POOL #1:      cryptonightv7.eu.nicehash.com:3363
 * API PORT:     4028
 * COMMANDS:     hashrate, pause, resume
[2018-05-18 09:24:57] compiling code and initializing GPUs. This will take a while...
[2018-05-18 09:24:57] #0, GPU #0 Radeon RX 580 Series, intensity: 0 (8/256), cu: 36
[2018-05-18 09:24:57] Error CL_INVALID_BUFFER_SIZE when calling clCreateBuffer to create hash scratchpads buffer.
[2018-05-18 09:24:57] Failed to start threads
```
Running with "--opencl-platform=1" (no devices set) runs fine:
```
 * VERSIONS:     XMRig/2.6.1 libuv/1.19.2 OpenCL/2.0 MSVC/2017
 * CPU:          Intel(R) Core(TM) i5-7600K CPU @ 3.80GHz x64 AES-NI
 * ALGO:         cryptonight, donate=1%
 * POOL #1:      cryptonightv7.eu.nicehash.com:3363
 * API PORT:     4028
 * COMMANDS:     hashrate, pause, resume
[2018-05-18 09:29:22] compiling code and initializing GPUs. This will take a while...
[2018-05-18 09:29:22] found AMD platform index: 1, name: Advanced Micro Devices, Inc.
[2018-05-18 09:29:22] found OpenCL GPU: Radeon RX 580 Series, cu: 36
[2018-05-18 09:29:22] #0, GPU #0 Radeon RX 580 Series, intensity: 864 (8/256), cu: 36
[2018-05-18 09:29:34] use pool cryptonightv7.eu.nicehash.com:3363 169.51.33.8
[2018-05-18 09:29:34] new job from cryptonightv7.eu.nicehash.com:3363 diff 100001
```
