Fix error handling threadz legacy profiles

A Go redefinition error causes pprof to skip parsing profile mappings.

profile/legacy_profile.go

 			break
 		}
 		// Ignore any unrecognized sections.
-		if l, err := b.ReadString('\n'); err != nil {
+		var err error
+		if l, err = b.ReadString('\n'); err != nil {
 			if err != io.EOF {
 				return err
 			}