msi: source one off error correction

[Mike McCormack]

Aric Stewart wrote:

>              rc = RegQueryValueExW(sourcekey, INSTALLPROPERTY_LASTUSEDSOURCEstringW,
>                      0, 0, (LPBYTE)buffer,&size); 
>              ptr = strchrW(buffer,';');
> +            ptr ++;
>              ptr = strchrW(ptr,';');
>              if (!ptr)
>                  rc = ERROR_UNKNOWN_PROPERTY;

This will crash immediately if there's anything other than a string with 
;; in it in the registry.

Mike