Fixed issue with walking beyond rule

examples/test.modal

@@ -1,4 +1,3 @@
-<> (?x dup)        (?x ?x)
-<> (?x ?y swap)     (?y ?x)
+<> (copy ?x) (?x ?x)
 
-(A B swap) dup
+(copy ABC)

src/modal.c

@@ -36,6 +36,7 @@ match(char *p, Rule *r)
 	char c, *a = r->a, *b = p;
 	while((c = *a)) {
 		if(c == '?') regs[(int)*(++a)] = b, a++, b = walk(b), c = *b;
+		if(!a[0]) return b;
 		if(c != *b) return NULL;
 		a++, b++;
 	}