fdisk: fix strict-aliasing bugs

fdisk: fix strict-aliasing bugs

gcc 4.4 produces tons of

  "dereferencing type-punned pointer will break strict-aliasing rules"

warnings for fdisk code where is

  char buffer[BUFSIZ];

  ((struct disklabel *) MBRBuffer)->foo

There are two ways how fix the problem:

  1/ union {
          char buffer[BUFSIZ], struct disklabel label
     } MBRBuffer;

  2/ use allocated buffer, this way seems less invasive.

This patch implements 2/.

Old version:
  $ make -C fdisk | grep -c warning
  236

New version:
  $ make -C fdisk | grep -c warning
  0

About aliasing:
 - http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40665
 - http://cellperformance.beyond3d.com/articles/2006/06/understanding-strict-aliasing.html
 - C99

Signed-off-by: Karel Zak <kzak@redhat.com>