Fix bug that crashes network with probability 2^-64 (#2358)

core/src/pow/types.rs

@@ -383,7 +383,7 @@ impl Proof {
 
 	/// Difficulty achieved by this proof with given scaling factor
 	fn scaled_difficulty(&self, scale: u64) -> u64 {
-		let diff = ((scale as u128) << 64) / (self.hash().to_u64() as u128);
+		let diff = ((scale as u128) << 64) / (max(1, self.hash().to_u64()) as u128);
 		min(diff, <u64>::max_value() as u128) as u64
 	}
 }