Fix OpenCL C code validation when multiple drivers are in use.

The first validation round consumed all standard input and empty
source was passed to subsequent rounds. We didn't notice this because
our secondary driver was POCL, which didn't complain about empty input
file.

Reported-by: VincentSC
Signed-off-by: Rami Ylimäki <rami.ylimaki@vincit.fi>