Fix segfault if pthread_getattr_np fails

glibc destroys[1] the passed pthread_attr_t if pthread_getattr_np()
fails.  Destroying it again leads to a segfault.  Fix it by only
destroying it on success for glibc.

[1]: https://sourceware.org/git/?p=glibc.git;a=blob;f=nptl/pthread_getattr_np.c;h=ce437205e41dc05653e435f6188768cccdd91c99;hb=HEAD#l205

library/std/src/sys/unix/thread.rs

@@ -305,7 +305,9 @@ pub mod guard {
             assert_eq!(libc::pthread_attr_getstack(&attr, &mut stackaddr, &mut stacksize), 0);
             ret = Some(stackaddr);
         }
-        assert_eq!(libc::pthread_attr_destroy(&mut attr), 0);
+        if e == 0 || cfg!(not(target_env = "gnu")) {
+            assert_eq!(libc::pthread_attr_destroy(&mut attr), 0);
+        }
         ret
     }
 
@@ -446,7 +448,9 @@ pub mod guard {
                 Some(stackaddr..stackaddr + guardsize)
             };
         }
-        assert_eq!(libc::pthread_attr_destroy(&mut attr), 0);
+        if e == 0 || cfg!(not(target_env = "gnu")) {
+            assert_eq!(libc::pthread_attr_destroy(&mut attr), 0);
+        }
         ret
     }
 }