Fix get_header_for_output for genesis (#2192)

chain/src/chain.rs

@@ -1139,6 +1139,9 @@ impl Chain {
 		loop {
 			let search_height = max - (max - min) / 2;
 			let h = txhashset.get_header_by_height(search_height)?;
+			if search_height == 0 {
+				return Ok(h);
+			}
 			let h_prev = txhashset.get_header_by_height(search_height - 1)?;
 			if pos > h.output_mmr_size {
 				min = search_height;