shm: Fix use of uninitialized value: segment's shared-memory type

shm: Fix use of uninitialized value: segment's shared-memory type

As shown by valgrind

  ==10615== Conditional jump or move depends on uninitialised value(s)
  ==10615==    at 0x5CC0483: shm_marker_size (shm.c:97)
  ==10615==    by 0x5CC1685: shm_attach (shm.c:381)
  ==10615==    by 0x5CC1990: pa_shm_cleanup (shm.c:453)
  ==10615==    by 0x5CC068E: sharedmem_create (shm.c:150)
  ...

Solution is to fix the shm_marker_size() signature itself: At
certain code paths like shm_attach(), we don't want to initialize
_any_ field in the passed SHM segment descriptor except after
making sure all error exit conditions have been passed.

Reported-by: Alexander E. Patrakov <patrakov@gmail.com>
Signed-off-by: Ahmed S. Darwish <darwish.07@gmail.com>
Signed-off-by: Arun Raghavan <arun@arunraghavan.net>