msvcrt: Avoid a NULL pointer deref in ctime.

dlls/msvcrt/tests/time.c

@@ -31,6 +31,13 @@
 #define MINSPERHOUR        60
 #define HOURSPERDAY        24
 
+static void test_ctime(void)
+{
+    time_t badtime = -1;
+    char* ret;
+    ret = ctime(&badtime);
+    ok(ret == NULL, "expected ctime to return NULL, got %s\n", ret);
+}
 static void test_gmtime(void)
 {
     time_t gmt = (time_t)NULL;
@@ -249,6 +256,7 @@ static void test_wstrtime(void)
 
 START_TEST(time)
 {
+    test_ctime();
     test_gmtime();
     test_mktime();
     test_localtime();

dlls/msvcrt/time.c

@@ -447,7 +447,10 @@ MSVCRT_wchar_t * CDECL MSVCRT__wasctime(const struct MSVCRT_tm *mstm)
  */
 char * CDECL MSVCRT_ctime(const MSVCRT_time_t *time)
 {
-    return MSVCRT_asctime( MSVCRT_localtime(time) );
+    struct MSVCRT_tm *t;
+    t = MSVCRT_localtime( time );
+    if (!t) return NULL;
+    return MSVCRT_asctime( t );
 }
 
 /*********************************************************************