Prevent crashes when calling deprecated functions

[kristopherjohnson]

Deprecated functions such as +[TBXML tbxmlWithXMLData:] were passing a null error pointer to other methods. This leads to a crash if an error occurs and the error-handling code doesn't check for a null pointer before setting the error.

These methods are deprecated, so callers shouldn't be using them, but legacy code should not crash.

This commit solves that problem by creating a local NSError reference and passing a pointer to that. For example:

- (id)initWithXMLData:(NSData*)aData {
    NSError *error = nil;
    return [self initWithXMLData:aData error:&error];
}

A test case has been added. This test case crashed before the fixes were made.