fix for following errors when compiling with visual studio (nw)

c:\emu\mamesvn\src\emu\netlist\pstring.h(26) : error C2220: warning treated as error - no 'object' file generated
c:\emu\mamesvn\src\emu\netlist\pstring.h(26) : warning C4200: nonstandard extension used : zero-sized array in struct/union
        Cannot generate copy-ctor or copy-assignment operator when UDT contains a zero-sized array
c:\emu\mamesvn\src\emu\netlist\pstring.h(171) : warning C4291: 'void *operator new(size_t,pblockpool &,int) throw(std::bad_alloc)' : no matching operator delete found; memory will not be freed if initialization throws an exception
        c:\emu\mamesvn\src\emu\netlist\pstring.h(51) : see declaration of 'operator new'