msvcrt: Rework wcsncpy_s to work on overlapping pointers.

dlls/msvcrt/tests/string.c

@@ -688,6 +688,16 @@ static void test_wcscpy_s(void)
     ok(ret == STRUNCATE, "expected STRUNCATE got %d\n", ret);
     ok(szDest[4] == 0, "szDest[4] not 0\n");
     ok(!memcmp(szDest, szLongText, 4*sizeof(WCHAR)), "szDest = %s\n", wine_dbgstr_w(szDest));
+
+    ret = p_wcsncpy_s(NULL, 0, (void*)0xdeadbeef, 0);
+    ok(ret == 0, "ret = %d\n", ret);
+
+    szDestShort[0] = '1';
+    szDestShort[1] = 0;
+    ret = p_wcsncpy_s(szDestShort+1, 4, szDestShort, -1);
+    ok(ret == STRUNCATE, "expected ERROR_SUCCESS got %d\n", ret);
+    ok(szDestShort[0]=='1' && szDestShort[1]=='1' && szDestShort[2]=='1' && szDestShort[3]=='1',
+            "szDestShort = %s\n", wine_dbgstr_w(szDestShort));
 }
 
 static void test__wcsupr_s(void)

dlls/msvcrt/wcs.c

@@ -1229,34 +1229,40 @@ INT CDECL MSVCRT_wcscpy_s( MSVCRT_wchar_t* wcDest, MSVCRT_size_t numElement, con
 INT CDECL MSVCRT_wcsncpy_s( MSVCRT_wchar_t* wcDest, MSVCRT_size_t numElement, const MSVCRT_wchar_t *wcSrc,
                             MSVCRT_size_t count )
 {
-    MSVCRT_size_t size = 0;
-    INT ret = 0;
+    WCHAR *p = wcDest;
+    BOOL truncate = (count == MSVCRT__TRUNCATE);
+
+    if(!wcDest && !numElement && !count)
+        return 0;
 
     if (!wcDest || !numElement)
         return MSVCRT_EINVAL;
 
-    wcDest[0] = 0;
-
     if (!wcSrc)
     {
+        *wcDest = 0;
         return count ? MSVCRT_EINVAL : 0;
     }
 
-    size = min(strlenW(wcSrc), count);
-    if (count==MSVCRT__TRUNCATE && size>=numElement)
+    while (numElement && count && *wcSrc)
     {
-        ret = MSVCRT_STRUNCATE;
-        size = numElement-1;
+        *p++ = *wcSrc++;
+        numElement--;
+        count--;
     }
-    else if (size >= numElement)
+    if (!numElement && truncate)
     {
+        *(p-1) = 0;
+        return MSVCRT_STRUNCATE;
+    }
+    else if (!numElement)
+    {
+        *wcDest = 0;
         return MSVCRT_ERANGE;
     }
 
-    memcpy( wcDest, wcSrc, size*sizeof(WCHAR) );
-    wcDest[size] = '\0';
-
-    return ret;
+    *p = 0;
+    return 0;
 }
 
 /******************************************************************