libcoprthr_opencl: clGetDeviceInfo for CL_DEVICE_EXTENSIONS causes segfault

When using libcopthr_opencl (through libocl), probing for device extensions with clGetDeviceInfo causes a segfault. Backtracing fingers the call to "strnlen" as the culprit, due to the "extensions" field of devinfo not being initialized.

Originally discovered trying to use the OpenCL backend for Octopus, but can be reproduced by extending clinfo.x in the test suite to probe device extensions as well.