win32: dirs: fix potential buffer overflow when appending "portable"

We let the concatenation decide if there's enough room or not.

And use the Wide char calls where wide char pointers are used.

src/win32/dirs.c

@@ -120,18 +120,21 @@ static char *config_GetAppDir (void)
 {
     /* if portable directory exists, use it */
     WCHAR path[MAX_PATH];
-    if (GetModuleFileName (NULL, path, MAX_PATH))
+    if (GetModuleFileNameW (NULL, path, MAX_PATH))
     {
-        WCHAR *lastDir = wcsrchr (path, TEXT('\\'));
+        WCHAR *lastDir = wcsrchr (path, L'\\');
         if (lastDir)
         {
-            wcscpy (lastDir + 1, TEXT("portable"));
+            *lastDir = L'\0';
-            DWORD attrib = GetFileAttributes (path);
+            if (wcscat_s(path, ARRAY_SIZE(path), TEXT("\\portable")) == 0)
+            {
+                DWORD attrib = GetFileAttributesW (path);
                 if (attrib != INVALID_FILE_ATTRIBUTES &&
                         (attrib & FILE_ATTRIBUTE_DIRECTORY))
                     return FromWide (path);
             }
         }
+    }
 
     char *psz_dir;
     char *psz_parent = config_GetKnownFolder (FOLDERID_RoamingAppData);