fix _bytes/nul-terminated for NULL results

Closes racket#2995

pkgs/racket-test-core/tests/racket/foreign-test.c

@@ -12,6 +12,8 @@ typedef unsigned char byte;
 #define X
 #endif
 
+X void* return_null() { return NULL; }
+
 X int  add1_int_int   (int  x) { return x + 1; }
 X int  add1_byte_int  (byte x) { return x + 1; }
 X byte add1_int_byte  (int  x) { return x + 1; }

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

@@ -575,6 +575,11 @@
 		  free)])
     (free p)))
 
+(let ([return_null (get-ffi-obj 'return_null test-lib (_fun -> _bytes/nul-terminated))])
+  (test #f return_null))
+(let ([return_null (get-ffi-obj 'return_null test-lib (_fun -> (_bytes/nul-terminated o 20)))])
+  (test #f return_null))
+
 ;; Test equality and hashing of c pointers:
 (let ([seventeen1 (cast 17 _intptr _pointer)]
       [seventeen2 (cast 17 _intptr _pointer)]

racket/collects/ffi/unsafe.rkt

@@ -1161,16 +1161,17 @@
 (define _bytes/nul-terminated
   (make-ctype _bytes
               (lambda (bstr) (and bstr (bytes-append bstr #"\0")))
-              (lambda (bstr) (bytes-copy bstr))))
+              (lambda (bstr) (and bstr (bytes-copy bstr)))))
 (provide (rename-out [_bytes/nul-terminated* _bytes/nul-terminated]))
 (define-fun-syntax _bytes/nul-terminated*
   (syntax-id-rules (o)
     [(_ o n) (type: _pointer
               pre:  (make-bytes n)
               ;; post is needed when this is used as a function output type
-              post: (x => (let ([s (make-bytes n)])
-                            (memcpy s x n)
-                            s)))]
+              post: (x => (and x
+                               (let ([s (make-bytes n)])
+                                 (memcpy s x n)
+                                 s))))]
     [(_ . xs) (_bytes/nul-teriminated . xs)]
     [_ _bytes/nul-terminated]))
 

racket/src/racket/src/salloc.c

@@ -3020,6 +3020,8 @@ Scheme_Object *scheme_dump_gc_stats(int c, Scheme_Object *p[])
 # endif
 #endif
 
+  scheme_console_printf("JIT-generated code: %ld\n", scheme_code_page_total);
+
 #if MZ_PRECISE_GC_TRACE
   if (for_each_struct) {
     scheme_console_printf("Begin Struct\n");