malloc: Fix a potential realloc issue with memory tagging

At an _int_free call site in realloc the wrong size was used for tag
clearing: the chunk header of the next chunk was also cleared which
in practice may work, but logically wrong.

The tag clearing is moved before the memcpy to save a tag computation,
this avoids a chunk2mem.  Another chunk2mem is removed because newmem
does not have to be recomputed. Whitespaces got fixed too.

Reviewed-by: DJ Delorie <dj@redhat.com>