runtime: x_cgo_getstackbound: initialize pthread attr

In glibc versions older than 2.32 (before commit 4721f95),
pthread_getattr_np does not always initialize the `attr` argument,
and when it fails, it results in a NULL pointer dereference in
pthread_attr_destroy down the road.

This is the simplest way to avoid this, and an alternative to CL 585019.

Updates #65625.

Change-Id: If490fd37020b03eb084ebbdbf9ae0248916426d0
Reviewed-on: https://go-review.googlesource.com/c/go/+/587919
Auto-Submit: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Cherry Mui <[email protected]>

src/runtime/cgo/gcc_stack_unix.c

@@ -18,20 +18,21 @@ x_cgo_getstackbound(uintptr bounds[2])
  void *addr;
  size_t size;
 
+ // Needed before pthread_getattr_np, too, since before glibc 2.32
+ // it did not call pthread_attr_init in all cases (see #65625).
+ pthread_attr_init(&attr);
 #if defined(__GLIBC__) || (defined(__sun) && !defined(__illumos__))
  // pthread_getattr_np is a GNU extension supported in glibc.
  // Solaris is not glibc but does support pthread_getattr_np
  // (and the fallback doesn't work...). Illumos does not.
  pthread_getattr_np(pthread_self(), &attr); // GNU extension
  pthread_attr_getstack(&attr, &addr, &size); // low address
 #elif defined(__illumos__)
- pthread_attr_init(&attr);
  pthread_attr_get_np(pthread_self(), &attr);
  pthread_attr_getstack(&attr, &addr, &size); // low address
 #else
  // We don't know how to get the current stacks, so assume they are the
  // same as the default stack bounds.
- pthread_attr_init(&attr);
  pthread_attr_getstacksize(&attr, &size);
  addr = __builtin_frame_address(0) + 4096 - size;
 #endif