H5Fopen with a non-ASCII filename causes assertion failure

On Windows Visual Studio debug build, calling H5Fopen with a filename that starts with non-ASCII character causes assertion failure at H5_CHECK_ABSOLUTE.
This is because isalpha is called with a negative number by sign extension. It should be masked with 0xff.
Similarly, other isCTYPE function calls should be masked, or else unexpected results may occur.