Use static memory instead of stack for OTA decoded data

* Default block size is 4096. Task stack size may have to be changed if
  allocating this buffer from stack. Using static memory here instead of
  stack

main/demo_tasks/ota_over_mqtt_demo/ota_over_mqtt_demo.c

@@ -1186,7 +1186,7 @@ static void processOTAEvents( void )
             }
             else
             {
-                uint8_t decodedData[ mqttFileDownloader_CONFIG_BLOCK_SIZE ];
+                static uint8_t decodedData[ mqttFileDownloader_CONFIG_BLOCK_SIZE ];
                 size_t decodedDataLength = 0;
                 MQTTFileDownloaderStatus_t xReturnStatus;
                 int16_t result = -1;