TextPop: do arithmetic before casting to pointer, not after

Clears warning from clang 13.0.1:

TextPop.c:679:64: warning: arithmetic on a null pointer treated as a cast
 from integer to pointer is a GNU extension [-Wnull-pointer-arithmetic]
    XtSetArg(args[num_args], XtNradioData, (XPointer)XawsdLeft + R_OFFSET);
                                           ~~~~~~~~~~~~~~~~~~~ ^

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Part-of: <!38>