Users: Check if the user ID passed as selected to wp_dropdown_users() corresponds to an existing user.

This avoids a few PHP notices if the `include_selected` parameter was specified and a non-existing user ID was passed.

Props campusboy1987.
Fixes #51370.

git-svn-id: https://develop.svn.wordpress.org/trunk@49036 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/user/wpDropdownUsers.php

@@ -129,6 +129,22 @@ class Tests_User_WpDropdownUsers extends WP_UnitTestCase {
 		$this->assertContains( $user1->user_login, $found );
 	}
 
+	/**
+	 * @ticket 51370
+	 */
+	public function test_include_selected_with_non_existing_user_id() {
+		$found = wp_dropdown_users(
+			array(
+				'echo'             => false,
+				'selected'         => PHP_INT_MAX,
+				'include_selected' => true,
+				'show'             => 'user_login',
+			)
+		);
+
+		$this->assertNotContains( (string) PHP_INT_MAX, $found );
+	}
+
 	/**
 	 * @ticket 38135
 	 */