restore revious presetToSave in case that mutex acquisition fails

wled00/presets.cpp

@@ -312,6 +312,7 @@ void savePreset(byte index, const char* pname, JsonObject sObj)
   }
 
   DEBUG_PRINT(F("Saving preset (")); DEBUG_PRINT(index); DEBUG_PRINT(F(") ")); DEBUG_PRINTLN(saveName);
+  auto oldpresetToSave = presetToSave; // for recovery in case that esp32SemTake(presetFileMux) fails
 
   presetToSave = index;
   playlistSave = false;
@@ -330,6 +331,7 @@ void savePreset(byte index, const char* pname, JsonObject sObj)
       // WLEDMM Acquire file mutex before writing presets.json, to prevent presets.json corruption
       if (esp32SemTake(presetFileMux, 2500) != pdTRUE) {
           USER_PRINTLN(F("savePreset(): preset file busy, cannot write"));
+          presetToSave = oldpresetToSave;
           return; // early exit, no change
       }