Fixed: 'dereference before null check' in DotIO.c

Fixed: 'dereference before null check' in DotIO.c

When the local variables in the function `convert_edge_labels_to_nodes` in
lib/spare/DotIO.c are initialized, the pointer `g` is dereferenced. But a
the check that tests if `g` is NULL happens after this initialization. The
fix is placing the `if (!g) return NULL` as the first line of the
function, so dereferencing only occurs if the pointer is not NULL.