when we check the parameter for upper(), lower() or len(),
return an error if the parameter is _no_ string

Bug: 10401
Change-Id: I5643ef05009072538155e63c3178071ed6bab061
Reviewed-on: https://code.wireshark.org/review/4071
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>