use stringbuilder instead of stringbuffer

src/com/jpexs/decompiler/flash/gui/Main.java

@@ -1517,7 +1517,7 @@ public class Main {
         try {
             java.security.MessageDigest md = java.security.MessageDigest.getInstance("MD5");
             byte[] array = md.digest(data);
-            StringBuffer sb = new StringBuffer();
+            StringBuilder sb = new StringBuilder();
             for (int i = 0; i < array.length; ++i) {
                 sb.append(Integer.toHexString((array[i] & 0xFF) | 0x100).substring(1, 3));
             }