expander: fix module-path?

Fix `module-path?` to check the first character of a path string.

Closes #1988

pkgs/racket-test-core/tests/racket/module.rktl

@@ -498,6 +498,13 @@
 (test #t module-path? "x/./foo.rkt")
 (test #t module-path? "x/.")
 (test #t module-path? "x/..")
+(test #f module-path? "@")
+(test #f module-path? "\0")
+(test #f module-path? "x@")
+(test #f module-path? "x\0")
+(test #f module-path? "@x")
+(test #f module-path? "\0x")
+
 
 (test #t module-path? (collection-file-path "module.rktl" "tests" "racket"))
 (test #t module-path? (string->path "x"))

racket/src/expander/common/parse-module-path.rkt

@@ -128,7 +128,7 @@
                      [saw-slash? (not file-end-ok?)]
                      [saw-dot? #f])
             (cond
-             [(not (zero? i))
+             [(not (negative? i))
               ;; check next character
               (define c (string-ref v i))
               (cond
@@ -314,7 +314,7 @@
   (define (test ok? v)
     (unless (equal? ok? (module-path? v))
       (error 'module-path?-test "failed ~s; expected ~a" v ok?)))
-  
+
   (test #t "hello")
   (test #t "hello.rkt")
   (test #f "hello*ss")
@@ -342,6 +342,13 @@
   (test #t "x/.")
   (test #t "x/..")
 
+  (test #f "@")
+  (test #f "\0")
+  (test #f "x@")
+  (test #f "x\0")
+  (test #f "@x")
+  (test #f "\0x")
+
   (test #t (collection-file-path "module.rktl" "tests" "racket"))
   (test #t (string->path "x"))
 

racket/src/racket/src/startup.inc

@@ -3093,7 +3093,7 @@ static const char *startup_source =
 "(lambda(i_26 prev-was-slash?_0 saw-slash?_0 saw-dot?_0)"
 "(begin"
 " 'loop"
-"(if(not(zero? i_26))"
+"(if(not(negative? i_26))"
 "(let-values()"
 "(let-values(((c_6)(string-ref v_36 i_26)))"
 "(if(char=? c_6 '#\\/)"