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]>

Author: kolyshkin

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