gpu.c: create_access_leaf: plug memory leak on error path
gpu.c: create_access_leaf: plug memory leak on error path

In particular, if the allocation of the isl_id referencing the ppcg_kernel_stmt
failed, then the ppcg_kernel_stmt does not get freed.
Free it explicitly.

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

Meinersbur/ppcg,Meinersbur/ppcg