Miner 2.13 cause BSOD at starting
Windows7, Driver v18.6.1, with only one AMD rx470

It was perfect with miner 2.8.1, but after I updated to 2.13. It keeps crashing and even BSOD. Beside pool and wallet, all config are default. I have compared the difference between 2.8.1 and 2.13. It seems 2.13 initials an extra device which 2.8.1 doesn't.

I tried to set `--opencl-devices=0` but it made thread starting fail.

```
 * ABOUT        XMRig-AMD/2.13.0 MSVC/2017
 * LIBS         libuv/1.24.1 OpenCL/2.0 OpenSSL/1.1.1a microhttpd/0.9.61
 * CPU          AMD Athlon(tm) II X4 640 Processor x64 -AES
 * ALGO         cryptonight, donate=5%
 * POOL #1      xmr-jp1.nanopool.org:14433 variant auto
 * COMMANDS     hashrate, pause, resume
[2019-03-05 14:29:50] compiling code and initializing GPUs. This will take a while...
[2019-03-05 14:29:50] found AMD platform index: 0, name: Advanced Micro Devices, Inc.
[2019-03-05 14:29:50] found OpenCL GPU: Radeon (TM) RX 470 Graphics (Ellesmere), cu: 32
[2019-03-05 14:29:50] #00, GPU #00 Radeon (TM) RX 470 Graphics (Ellesmere), i:768 (8/256), si:2/2, u:8, cu:32
[2019-03-05 14:29:50]              1.50/3.75/4 GB
[2019-03-05 14:29:50] GPU #0 compiling...
[2019-03-05 14:30:21] GPU #0 compilation completed, elapsed time 30.976s
[2019-03-05 14:30:21] #01, GPU #00 Radeon (TM) RX 470 Graphics (Ellesmere), i:768 (8/256), si:2/2, u:8, cu:32
[2019-03-05 14:30:21]              1.50/3.75/4 GB
[2019-03-05 14:30:21] GPU #0 compiling...
[2019-03-05 14:30:52] GPU #0 compilation completed, elapsed time 31.004s
[2019-03-05 14:30:52] speed 10s/60s/15m n/a n/a n/a H/s max n/a H/s
```