gpu.c: create_kernel_vars: avoid invalid access on error path
gpu.c: create_kernel_vars: avoid invalid access on error path

In particular, keep n_var set to zero until after a successful
allocation.

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

Meinersbur/ppcg,Meinersbur/ppcg