[C API] Missing error check on table_x_copy

[jeromekelleher]

e.g.

tskit/c/tskit/tables.c

Lines 1410 to 1413 in 5f58b61

	ret = tsk_edge_table_set_columns(dest, self->num_rows, self->left, self->right,
	self->parent, self->child, self->metadata, self->metadata_offset);
	ret = tsk_edge_table_set_metadata_schema(
	dest, self->metadata_schema, self->metadata_schema_length);