Optimised use of buffers when decrypting to avoid unnecessary array copying

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

@@ -156,6 +156,12 @@ public class DecryptingPartInputStream extends FileInputStream {
 
       //data retrieved using cipher.update doesn't always match cipher.getOutputSize (but should never be larger)
       int outputLen = cipher.getOutputSize(read);
+
+      if (outputLen < length) {
+        readLength += cipher.update(internalBuffer, 0, read, buffer, offset);
+        return readLength;
+      }
+
       byte[] transientBuffer = new byte[outputLen];
       outputLen = cipher.update(internalBuffer, 0, read, transientBuffer, 0);
       if (outputLen <= length) {