Fix Skip::nth

1. Remove an unnecessary condition.
2. Avoid advancing after nth/next returns None.

Author: Stebalien

src/libcore/iter.rs

@@ -2039,14 +2039,17 @@ impl<I> Iterator for Skip<I> where I: Iterator {
 
     #[inline]
     fn nth(&mut self, n: usize) -> Option<I::Item> {
+        // Can't just add n + self.n due to overflow.
         if self.n == 0 {
             self.iter.nth(n)
-        } else if n == 0 {
-            self.next()
         } else {
-            self.next();
-            // Recurse on the first case.
-            self.iter.nth(n-1)
+            let to_skip = self.n;
+            self.n = 0;
+            // nth(n) skips n+1
+            if self.iter.nth(to_skip-1).is_none() {
+                return None;
+            }
+            self.iter.nth(n)
         }
     }