Use brackets around shift operations

Use `( x >> y ) & z` instead of `x >> y & z`. Both are equivalent
by operator precedence, but the former is more readable and the
commonly used idiom in the library.
diff --git a/library/ssl_ticket.c b/library/ssl_ticket.c
index ad2d526..e80df28 100644
--- a/library/ssl_ticket.c
+++ b/library/ssl_ticket.c
@@ -192,9 +192,9 @@
     if( left < 3 + cert_len )
         return( MBEDTLS_ERR_SSL_BUFFER_TOO_SMALL );
 
-    *p++ = (unsigned char)( cert_len >> 16 & 0xFF );
-    *p++ = (unsigned char)( cert_len >>  8 & 0xFF );
-    *p++ = (unsigned char)( cert_len       & 0xFF );
+    *p++ = (unsigned char)( ( cert_len >> 16 ) & 0xFF );
+    *p++ = (unsigned char)( ( cert_len >>  8 ) & 0xFF );
+    *p++ = (unsigned char)( ( cert_len       ) & 0xFF );
 
     if( session->peer_cert != NULL )
         memcpy( p, session->peer_cert->raw.p, cert_len );