CID 1292483 Uninitialized pointer read (UNINIT)
Initialize the method and credential members of the opal_sec_cred_t to
avoid possible invalid read when calling cleanup_cred.
CID 1292484 Double free (USE_AFTER_FREE)
Set method and credential members to NULL after freeing in
cleanup_cred.
Signed-off-by: Nathan Hjelm <hjelmn@lanl.gov>