sim/rx: avoid pointer arithmetic on void * pointers
Pointer arithmetic on void * pointers results in a GCC warning. Avoid
the warning by casting the pointer to its actual type earlier in the
function.
sim/rx/ChangeLog:
* mem.c (mem_put_blk): Rename parameter, add cast from parameter
type to local type. Remove cast later in the function.
(mem_get_blk): Likewise.
* mem.h (mem_put_blk): Rename parameter to match definition.
(mem_get_blk): Likewise.