clGetPlatformIDs return value fix

It should return CL_SUCCESS in case num_platforms == NULL && num_entries
== 0.  At least Glow checks for availability of OpenCL (in general)
using these parameters.

Specs say:
"If platforms is not NULL, the num_entries must be greater than zero."

lib/CL/clGetPlatformIDs.c

@@ -251,7 +251,7 @@ POname(clGetPlatformIDs)(cl_uint           num_entries,
                           CL_INVALID_VALUE);
 
   POCL_RETURN_ERROR_COND ((num_platforms == NULL && num_entries == 0),
-                          CL_INVALID_VALUE);
+                          CL_SUCCESS);
 
   if (platforms != NULL) {
       platforms[0] = &_platforms[0];