Fix compiler warnings seen with clang 8.1.0 on MacOS

This fixes the following warnings:

test_tcp.c:266:5: error: code will never be executed [-Werror,-Wunreachable-code]
    pbuf_free(p);
    ^~~~~~~~~
 - The check API 'fail' aborts the test, thus pbuf_free(p) will never be executed

pbuf.c:783:111: error: format specifies type 'unsigned short' but the argument has type 'u8_t' (aka 'unsigned char') [-Werror,-Wformat]
      LWIP_DEBUGF( PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_free: %p has ref %"U16_F", ending here.\n", (void *)p, ref));
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
 - LWIP_PBUF_REF_T is u8_t by default and doesn't match U16_F, so cast to u16_t. The cast and formatter will need to be changed
   if ref is larger than 16 bits

ethernet.c:105:16: error: format specifies type 'unsigned char' but the argument has type 'unsigned int' [-Werror,-Wformat]
               (unsigned)ethhdr->dest.addr[0], (unsigned)ethhdr->dest.addr[1], (unsigned)ethhdr->dest.addr[2],
~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 - addr[] is type u8_t, formatter is X8_F which should be 8 bits. 'unsigned' is an int, so cast to unsighed char instead

test/unit/tcp/test_tcp.c

@@ -263,7 +263,6 @@ test_tcp_recv_expectclose(void* arg, struct tcp_pcb* pcb, struct pbuf* p, err_t
 
   if (p != NULL) {
     fail();
-    pbuf_free(p);
   } else {
     /* correct: FIN received; close our end, too */
     err_t err2 = tcp_close(pcb);