util/all_gpus.cpp: throw custom exception instead of SEGV in select_g…

…pu().

Application is expected to check for GPU availability prior to making any
calls, including the one in quesiton. In other word it shouldn't get here,
but if it did, make the failure more informative.

util/all_gpus.cpp

@@ -31,6 +31,8 @@ class gpus_t {
 const gpu_t& select_gpu(int id)
 {
     auto& gpus = gpus_t::all();
+    if (gpus.size() == 0)
+        CUDA_OK(cudaErrorNoDevice);
     if (id == -1) {
         int cuda_id;
         CUDA_OK(cudaGetDevice(&cuda_id));