When an OpenCL platform has no devices (of the requested type), calls to
clGetDeviceIDs() are required to return with the error value
CL_DEVICE_NOT_FOUND.

Some platforms (e.g. Clover as of Mesa 10.4.2) do not touch their output
parameters in such cases, which means that in some conditions the `num`
variable where the number of devices of the platform should be stored
may be used uninitialized. This can lead to segmentations faults in the
subsequent calls to clGetDeviceInfo().

Simply reinitializing num to 0 is sufficient to prevent the segfault in
the case of Mesa, but proper error handling is included for
completeness.

Change-Id: Ia25192f6aa953838a545a9e7c9fca050d2703b60
Reviewed-on: https://gerrit.libreoffice.org/14700Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tor Lillqvist <tml@collabora.com>