Use xmalloc and xrealloc, rather than unchecked malloc and realloc.

Use xmalloc and xrealloc, rather than unchecked malloc and realloc.
* parted/table.c: Use gnulib's xmalloc and realloc.
Remove anachronistic casts of malloc/realloc return value.
Change "sizeof(type)" to safer "sizeof(*var)" (one of the former
was wrong, but in a harmless way).