winpr: add some functions to use wStream in a static way

winpr: add some functions to use wStream in a static way

It's sometime useful to create a stream aliasing a buffer on the stack, and
it's nice if we don't need some extra malloc for this.

Example use:
   BYTE buffer[20];
   wStream s;

   Stream_StaticInit(&s, buffer, sizeof(buffer));
   Stream_Write_UINT16(&s, 0xff01);
   Stream_Free(&s, FALSE);