Address grumbles in eras PR (#782)

* Address grumbles

* Fix hash

substrate/core/sr-primitives/src/generic/era.rs

@@ -79,7 +79,7 @@ impl Era {
 	pub fn birth(self, current: u64) -> u64 {
 		match self {
 			Era::Immortal => 0,
-			Era::Mortal(period, phase) => (current - phase) / period * period + phase,
+			Era::Mortal(period, phase) => (current.max(phase) - phase) / period * period + phase,
 		}
 	}
 
@@ -189,4 +189,10 @@ mod tests {
 		assert_ne!(e.birth(10), 6);
 		assert_ne!(e.birth(5), 6);
 	}
+	
+	#[test]
+	fn current_less_than_phase() {
+		// should not panic
+		Era::mortal(4, 3).birth(1);
+	}
 }