user.exe16: Fix an incorrect use of ADD_TO_T in FormatMessage16.

[Henri Verbeet]

On 9 July 2017 at 18:53, Gerald Pfeifer <gerald at pfeifer.com> wrote:
> ADD_TO_T actually consists of two statements
>
>   #define ADD_TO_T(c) \
>         *t++=c;\
>         if (t-target == talloced) {\
>                 target  = HeapReAlloc(GetProcessHeap(),HEAP_ZERO_MEMORY,target,talloced*2);\
>                 t       = target+talloced;\
>                 talloced*=2;\
>         }
>
> so
>
>   for (x=b; *x; x++) ADD_TO_T(*x);
>
> does _not_ actually do what one might expect/hope.
>
> Fixed thusly, in line with other usage.
>
Alternatively, it shouldn't be hard to either fix the macro or replace
it with a function.