Don't append garbage to the function parameter slots

If the argument is neither a temporary nor a constant, slot isn't
initialized. So don't try to add it.

qqmltypecompiler.cpp:2548:36: error: variable 'slot' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
qqmltypecompiler.cpp:2553:36: note: uninitialized use occurs here
        _functionParameters.append(slot);
                                   ^~~~

Change-Id: I5a538bfa0ef1fbbd00e23dfd6e15c404359c7c46
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>