Fix error prone check in LibraryDesugaringOptions

Change-Id: If0223877a073caf91b21ba8efb998c0cacfcc33f
diff --git a/src/main/java/com/android/tools/r8/ir/desugar/desugaredlibrary/LibraryDesugaringOptions.java b/src/main/java/com/android/tools/r8/ir/desugar/desugaredlibrary/LibraryDesugaringOptions.java
index 1ba58e8..f5b54e0 100644
--- a/src/main/java/com/android/tools/r8/ir/desugar/desugaredlibrary/LibraryDesugaringOptions.java
+++ b/src/main/java/com/android/tools/r8/ir/desugar/desugaredlibrary/LibraryDesugaringOptions.java
@@ -18,7 +18,7 @@
   private MachineDesugaredLibrarySpecification machineDesugaredLibrarySpecification =
       MachineDesugaredLibrarySpecification.empty();
   private String synthesizedClassPrefix = "";
-  private DesugaredLibraryTypeRewriter typeRewriter;
+  private volatile DesugaredLibraryTypeRewriter typeRewriter;
 
   private final InternalOptions options;