Dev fix: _pluck should check that what is is accessing exists

- When using _pluck with deferred rendering, there were a number of
  errors due to the nTr element being undefined (that's what deferred
  rendering is all about of course). So the logic to check that pluck
  can pluck from a source object needs ot be added.

media/src/api/api._selectors.js

@@ -9,14 +9,18 @@ var _pluck = function ( a, prop, prop2 ) {
 	// is essential here
 	if ( prop2 !== undefined ) {
 		for ( ; i<ien ; i++ ) {
+			if ( a[i] && a[i][ prop ] ) {
 				out.push( a[i][ prop ][ prop2 ] );
 			}
 		}
+	}
 	else {
 		for ( ; i<ien ; i++ ) {
+			if ( a[i] ) {
 				out.push( a[i][ prop ] );
 			}
 		}
+	}
 
 	return out;
 };