Add synchronized access to cache

src/org/thoughtcrime/securesms/crypto/MessageDisplayHelper.java

@@ -34,6 +34,7 @@ public class MessageDisplayHelper {
   };
 
   private static String checkCacheForBody(String body) {
+    synchronized (decryptedBodyCache) {
       if (decryptedBodyCache.containsKey(body)) {
         String decryptedBody = decryptedBodyCache.get(body).get();
         if (decryptedBody != null) {
@@ -46,6 +47,7 @@ public class MessageDisplayHelper {
 
       return null;
     }
+  }
 
   public static String getDecryptedMessageBody(MasterCipher bodyCipher, String body) throws InvalidMessageException {
     if (body.startsWith(Prefix.SYMMETRIC_ENCRYPT)) {
@@ -55,7 +57,10 @@ public class MessageDisplayHelper {
         return cacheResult;
 
       String decryptedBody = bodyCipher.decryptBody(body.substring(Prefix.SYMMETRIC_ENCRYPT.length()));
+
+      synchronized (decryptedBodyCache) {
         decryptedBodyCache.put(body, new SoftReference<String>(decryptedBody));
+      }
 
       return decryptedBody;
     }