rustbuild: Less panics in musl_root

Don't panic if the target wasn't configured.

src/bootstrap/lib.rs

@@ -991,7 +991,8 @@ impl Build {
 
     /// Returns the "musl root" for this `target`, if defined
     fn musl_root(&self, target: &str) -> Option<&Path> {
-        self.config.target_config[target].musl_root.as_ref()
+        self.config.target_config.get(target)
+            .and_then(|t| t.musl_root.as_ref())
             .or(self.config.musl_root.as_ref())
             .map(|p| &**p)
     }