msvcrt: Fix _dupenv_s behavior with missing env var.

dlls/msvcr80/tests/msvcr80.c

@@ -197,11 +197,8 @@ static void test_dupenv_s(void)
     len = 0xdeadbeef;
     tmp = (void *)0xdeadbeef;
     ret = p_dupenv_s( &tmp, &len, "nonexistent" );
-    todo_wine
     ok( !ret, "_dupenv_s returned %d\n", ret );
-    todo_wine
     ok( !len, "_dupenv_s returned length is %Id\n", len );
-    todo_wine
     ok( !tmp, "_dupenv_s returned pointer is %p\n", tmp );
 }
 

dlls/msvcrt/environ.c

@@ -249,7 +249,12 @@ int CDECL _dupenv_s(char **buffer, size_t *numberOfElements, const char *varname
     if (!MSVCRT_CHECK_PMT(buffer != NULL)) return EINVAL;
     if (!MSVCRT_CHECK_PMT(varname != NULL)) return EINVAL;
 
-    if (!(e = getenv(varname))) return *_errno() = EINVAL;
+    if (!(e = getenv(varname)))
+    {
+        *buffer = NULL;
+        if (numberOfElements) *numberOfElements = 0;
+        return 0;
+    }
 
     sz = strlen(e) + 1;
     if (!(*buffer = malloc(sz)))