Respect TOSTRAND() lifetime

In a different context, I questioned my understanding of C object
lifetimes and learned that I had given the wrong explanation for why
TOSTRAND() was used wrongly in
0c96fc65:

TOSTRAND() creates a compound literal whose lifetime is the enclosing
block.

Thus, the error was unrelated to temporary lifetime, but rather that
the compound literal was assigned in the if-block, but used outside
it.

Ref: https://stackoverflow.com/questions/71225472/more-questions-on-the-c11-temporary-lifetime-rule-and-undefined-behaviour/71226049#71226049

vmod/vmod_vtc.c

@@ -474,10 +474,9 @@ vsl_line(VRT_CTX, char *str)
 		return;
 
 	str = strtok_r(NULL, "\r\n", &save);
+	s = TOSTRAND(str);
 	if (str == NULL)
 		s = vrt_null_strands;
-	else
-		s = TOSTRAND(str);
 
 	vmod_vsl(ctx, id, tag, side, s);
 }