msvcrt: Fix handling of dst == src in wcscpy_s.

dlls/msvcrt/tests/string.c

@@ -1020,6 +1020,11 @@ static void test_wcscpy_s(void)
     ok(ret == 0, "expected 0 got %d\n", ret);
     ok(lstrcmpW(szDest, szLongText) == 0, "szDest != szLongText\n");
 
+    /* dest == source */
+    ret = p_wcscpy_s(szDest, 18, szDest);
+    ok(ret == 0, "expected 0 got %d\n", ret);
+    ok(lstrcmpW(szDest, szLongText) == 0, "szDest != szLongText\n");
+
     /* Copy smaller buffer size */
     errno = EBADF;
     szDest[0] = 'A';

dlls/msvcrt/wcs.c

@@ -2041,16 +2041,21 @@ INT CDECL MSVCRT_wcscpy_s( MSVCRT_wchar_t* wcDest, MSVCRT_size_t numElement, con
     if(!MSVCRT_CHECK_PMT(wcDest)) return MSVCRT_EINVAL;
     if(!MSVCRT_CHECK_PMT(numElement)) return MSVCRT_EINVAL;
 
-    wcDest[0] = 0;
-
-    if(!MSVCRT_CHECK_PMT(wcSrc)) return MSVCRT_EINVAL;
+    if(!MSVCRT_CHECK_PMT(wcSrc))
+    {
+        wcDest[0] = 0;
+        return MSVCRT_EINVAL;
+    }
 
     size = strlenW(wcSrc) + 1;
 
     if(!MSVCRT_CHECK_PMT_ERR(size <= numElement, MSVCRT_ERANGE))
+    {
+        wcDest[0] = 0;
         return MSVCRT_ERANGE;
+    }
 
-    memcpy( wcDest, wcSrc, size*sizeof(WCHAR) );
+    memmove( wcDest, wcSrc, size*sizeof(WCHAR) );
 
     return 0;
 }