Fixed incorrect reference to decrypted block

Fixed incorrectly copied memory from p->buf into init instead of from the decrypted buffer block. The only reason this worked was because the crypt() function decrypts the value in-place and overwrites p->buf.  I'm working on a fork that no longer does this and exposed this bug.

src/transport.c

@@ -420,7 +420,7 @@ int _libssh2_transport_read(LIBSSH2_SESSION * session)
                 }
                 /* save the first 5 bytes of the decrypted package, to be
                    used in the hash calculation later down. */
-                memcpy(p->init, &p->buf[p->readidx], 5);
+                memcpy(p->init, block, 5);
             } else {
                 /* the data is plain, just copy it verbatim to
                    the working block buffer */