Fix array-bounds compiler warning on gcc11+ in list.h (#580)

listGET_OWNER_OF_NEXT_ENTRY computes `( pxConstList )->pxIndex->pxNext` after
verifying that `( pxConstList )->pxIndex` points to `xListEnd`, which due to
being a MiniListItem_t, can be shorter than a ListItem_t. Thus,
`( pxConstList )->pxIndex` is a `ListItem_t *` that extends past the end of the
`List_t` whose `xListEnd` it points to. This is fixed by accessing `pxNext`
through a `MiniListItem_t` instead.

include/list.h

@@ -290,7 +290,7 @@ typedef struct xLIST
         ( pxConstList )->pxIndex = ( pxConstList )->pxIndex->pxNext;                           \
         if( ( void * ) ( pxConstList )->pxIndex == ( void * ) &( ( pxConstList )->xListEnd ) ) \
         {                                                                                      \
-            ( pxConstList )->pxIndex = ( pxConstList )->pxIndex->pxNext;                       \
+            ( pxConstList )->pxIndex = ( pxConstList )->xListEnd.pxNext;                       \
         }                                                                                      \
         ( pxTCB ) = ( pxConstList )->pxIndex->pvOwner;                                         \
     }