mdadm: Assemble.c fix coverity issues

mdadm: Assemble.c fix coverity issues

Fixing the following coding errors the coverity tools found:

* Event dereference: Dereferencing "pre_exist", which is known to be "NULL".
* Event parameter_hidden: Declaration hides parameter "c".
* Event leaked_storage: Variable "pre_exist" going out of scope leaks the
  storage it points to.
* Event leaked_storage: Variable "avail" going out of scope leaks the
  storage it points to.

Signed-off-by: Nigel Croxon <ncroxon@redhat.com>