one-true-awk: Avoid a NULL dereference.

CID:		270862
Obtained from:	NetBSD (CVS Rev. 1.8)
MFC after:	2 weeks

contrib/one-true-awk/tran.c

@@ -333,7 +333,7 @@ char *setsval(Cell *vp, const char *s)	/* set string val of a Cell */
 		donefld = 0;	/* mark $1... invalid */
 		donerec = 1;
 	}
-	t = tostring(s);	/* in case it's self-assign */
+	t = s ? tostring(s) : tostring("");	/* in case it's self-assign */
 	if (freeable(vp))
 		xfree(vp->sval);
 	vp->tval &= ~NUM;