Reapply r301691:

Revert r301689 - one-true-awk: Avoid a NULL dereference.

I got this wrong and the coverity report doesn't match the NetBSD change,
which was thought for a different version.

The change wouldn't hurt but let's wait until upstream figures this out.

contrib/one-true-awk/tran.c

@@ -366,7 +366,7 @@ char *setsval(Cell *vp, const char *s)	/* set string val of a Cell */
 		if (donerec == 0)
 			recbld();
 	}
-	t = s ? tostring(s) : tostring("");	/* in case it's self-assign */
+	t = tostring(s);	/* in case it's self-assign */
 	if (freeable(vp))
 		xfree(vp->sval);
 	vp->tval &= ~(NUM|CONVC|CONVO);