Use the proper method to update a signature with salt

Fixes #1780

Using update() instead of sigOut.write() caused some preprocessing on the salt value
in some cases (Cleartext Signature Framework line endings), which resulted in broken
signature verification.

Author: vanitasvitae

pg/src/main/java/org/bouncycastle/openpgp/PGPSignature.java

@@ -195,10 +195,18 @@ private void checkSaltSize()
     }
 
     private void updateWithSalt()
+            throws PGPException
     {
         if (getVersion() == SignaturePacket.VERSION_6)
         {
-            update(sigPck.getSalt());
+            try
+            {
+                sigOut.write(sigPck.getSalt());
+            }
+            catch (IOException e)
+            {
+                throw new PGPException("Could not update with salt.", e);
+            }
         }
     }