sim/rx: avoid pointer arithmetic on void * pointers

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.