Tweak parse_opt_uint.

Don't set `slot` on failure, like all the other `parse_*` functions.

src/librustc_session/options.rs

@@ -421,7 +421,7 @@ fn parse_uint(slot: &mut usize, v: Option<&str>) -> bool {
         fn parse_opt_uint(slot: &mut Option<usize>, v: Option<&str>) -> bool {
             match v {
                 Some(s) => { *slot = s.parse().ok(); slot.is_some() }
-                None => { *slot = None; false }
+                None => false
             }
         }