Avoid race condition. Following the left to right rule,

s.output.Data() was being retrieved before the synchronization
point, which meant that it could be retrieved before the
goroutine wrote it.  Using gccgo this caused random errors.

R=gri
DELTA=2  (1 added, 0 deleted, 1 changed)
OCL=31046
CL=31267

src/pkg/datafmt/datafmt.go

@@ -745,7 +745,8 @@ func (f Format) Eval(env Environment, args ...) ([]byte, os.Error) {
 		errors <- nil;  // no errors
 	}();
 
-	return s.output.Data(), <- errors;
+	err := <- errors;
+	return s.output.Data(), err;
 }