Fix string-ref bug.

Author: dvanhorn

langs/hoax/compile-ops.rkt

@@ -240,6 +240,8 @@
      (seq (Pop r8)
           (assert-string r8)
           (assert-integer rax)
+          (Cmp r8 type-str)
+          (Je 'raise_error_align) ; special case for empty string
           (Cmp rax 0)
           (Jl 'raise_error_align)
           (Xor r8 type-str)       ; r8 = ptr

langs/hoax/test/test-runner.rkt

@@ -145,6 +145,8 @@
   (check-equal? (run '(make-string 3 #\g)) "ggg")
   (check-equal? (run '(string-length "")) 0)
   (check-equal? (run '(string-length "fred")) 4)
+  (check-equal? (run '(string-ref "" 0)) 'err)
+  (check-equal? (run '(string-ref (make-string 0 #\a) 0)) 'err)
   (check-equal? (run '(string-ref "fred" 0)) #\f)
   (check-equal? (run '(string-ref "fred" 1)) #\r)
   (check-equal? (run '(string-ref "fred" 2)) #\e)