Fix zero-size memory block issues in OpenCL backend.
Fix zero-size memory block issues in OpenCL backend.

HIPERFIT/futhark,ihc/futhark,mrakgr/futhark,diku-dk/futhark,diku-dk/futhark,mrakgr/futhark,mrakgr/futhark,HIPERFIT/futhark,ihc/futhark,CulpaBS/wbBach,diku-dk/futhark,diku-dk/futhark,HIPERFIT/futhark,diku-dk/futhark,ihc/futhark