runtime/cgo: add assertions for safe stack retrieval

Author: froz42

src/runtime/cgo/gcc_stack_unix.c

@@ -8,6 +8,7 @@
 #define _GNU_SOURCE
 #endif
 
+#include <assert.h>
 #include <pthread.h>
 #include "libcgo.h"
 
@@ -20,16 +21,16 @@ x_cgo_getstackbound(uintptr bounds[2])
 
 	// 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);
+	assert(pthread_attr_init(&attr) == 0);
 #if defined(__GLIBC__) || defined(__BIONIC__) || (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
+	assert(pthread_getattr_np(pthread_self(), &attr) == 0); // GNU extension
+	assert(pthread_attr_getstack(&attr, &addr, &size) == 0); // low address
 #elif defined(__illumos__)
-	pthread_attr_get_np(pthread_self(), &attr);
-	pthread_attr_getstack(&attr, &addr, &size); // low address
+	assert(pthread_attr_get_np(pthread_self(), &attr) == 0); // Solaris extension
+	assert(pthread_attr_getstack(&attr, &addr, &size) == 0); // low address
 #else
 	// We don't know how to get the current stacks, leave it as
 	// 0 and the caller will use an estimate based on the current