REST API: Fix incorrect uses of rest_sanitize_value_from_schema().

In the `check_username()` and `check_password()` callbacks in the Users controller cast the provided request value to a string. The `rest_sanitize_value_from_schema()` function was being used incorrectly which was causing unintended request parsing. 
In `rest_sanitize_request_arg()` do not pass nonexistent third parameter for the `rest_sanitize_value_from_schema()` function.

Props jnylen0, joehoyle, rachelbaker, ocean90.
Fixes #38984.

git-svn-id: https://develop.svn.wordpress.org/trunk@39400 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/rest-api.php

@@ -840,7 +840,7 @@ function rest_sanitize_request_arg( $value, $request, $param ) {
 	}
 	$args = $attributes['args'][ $param ];
 
-	return rest_sanitize_value_from_schema( $value, $args, $param );
+	return rest_sanitize_value_from_schema( $value, $args );
 }
 
 /**