Use void * for pointers in with_echo_area_buffer

Use void * for pointers in with_echo_area_buffer

* src/xdisp.c (with_echo_area_buffer): Pass void * instead of
ptrdiff_t, since the values are typically pointers and this ports
better to (mostly-theoretical) hosts where ptrdiff_t is narrower
than intptr_t.  All uses changed.