Fix use of __pthread_attr_copy in mq_notify (bug 27896)

__pthread_attr_copy can fail and does not initialize the attribute
structure in that case.

If __pthread_attr_copy is never called and there is no allocated
attribute, pthread_attr_destroy should not be called, otherwise
there is a null pointer dereference in rt/tst-mqueue6.

Fixes commit 42d35935
("Use __pthread_attr_copy in mq_notify (bug 27896)").

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
(cherry picked from commit 217b6dc2)