fix(uart): add check for malloc in uart examples

Closes #13405

Author: songruo

examples/peripherals/uart/uart_echo_rs485/main/rs485_example.c

@@ -94,6 +94,7 @@ static void echo_task(void *arg)
 
     // Allocate buffers for UART
     uint8_t* data = (uint8_t*) malloc(BUF_SIZE);
+    assert(data);
 
     ESP_LOGI(TAG, "UART start receive loop.\r");
     echo_send(uart_num, "Start RS485 UART test.\r\n", 24);

examples/peripherals/uart/uart_events/main/uart_events_example_main.c

@@ -41,6 +41,7 @@ static void uart_event_task(void *pvParameters)
     uart_event_t event;
     size_t buffered_size;
     uint8_t* dtmp = (uint8_t*) malloc(RD_BUF_SIZE);
+    assert(dtmp);
     for (;;) {
         //Waiting for UART event.
         if (xQueueReceive(uart0_queue, (void *)&event, (TickType_t)portMAX_DELAY)) {