Remove unnecessary `current_block &&`

[Rich-Harris]

Very minor thing I noticed while working on #525 — if you have an if block (or a chain of if, elseif, elseif...) followed by an else block, then there's no need to check for the existence of current_block:

function get_block ( state ) {
  if ( state.foo ) return create_if_block;
  return create_if_block_1;
}

var current_block = get_block( state );
-var if_block = current_block && current_block( state, component );
+var if_block = current_block( state, component );

return {
  mount: function ( target, anchor ) {
    // ...
  },

  update: function ( changed, state ) {
    // ...

    if ( current_block !== ( current_block = get_block( state ) ) ) {
-      if ( if_block ) if_block.destroy( true );
-      if_block = current_block && current_block( state, component );
-      if ( if_block ) if_block.mount( if_block_anchor.parentNode, if_block_anchor );
+      if_block.destroy( true );
+      if_block = current_block( state, component );
+      if_block.mount( if_block_anchor.parentNode, if_block_anchor );
    }
  },

  destroy: function ( detach ) {
    // ...
-    if ( if_block ) if_block.destroy( detach );
+    if_block.destroy( detach );
  }
};

This'll have to wait until after #525 in order to avoid merge conflicts.