rustfmt: gate assigning style-edition on nightly as well

The `style-edition` is declared as a nightly option, but later we
attempt to read the value of the `style-edition` even when we are not on
the nightly channel, causing a panic on e.g. beta.

Author: jieyouxu

src/tools/rustfmt/src/bin/main.rs

@@ -654,8 +654,10 @@ impl GetOptsOptions {
             }
         }
 
-        if let Some(ref edition_str) = matches.opt_str("style-edition") {
-            options.style_edition = Some(style_edition_from_style_edition_str(edition_str)?);
+        if is_nightly() {
+            if let Some(ref edition_str) = matches.opt_str("style-edition") {
+                options.style_edition = Some(style_edition_from_style_edition_str(edition_str)?);
+            }
         }
 
         Ok(options)