vsdg: throw error for input/output index out of bound
vsdg: throw error for input/output index out of bound

Fix jive_node_input and jive_node_output to throw error instead of
returning NULL when the index is out of bounds.

Signed-off-by: Helge Bahmann <c5618920f622561f9c0ec5c42219c4c08180149f@chaoticmind.net>

phate/jive,phate/jive,phate/jive