Patch for bug #2070708 (WinVer.nsh, service pack macros)

Hi all,

This is a corrected version of __GetWinServicePack macro (WinVer.nsh)
This macro is supposed to be used as a condition for ${If...} statements, therefore it should not contain logic-lib stuff itself. (For some reason, nested IF-s are not supported...)
I've modified the macro to use local jumps instead of If/Else statements.

Enjoy!