Avoid null pointer dereference to implement container_of

Using offsetof instead of actually dereferencing a 0 pointer is still
standards compliant (works under gcc -ansi, clang -ansi), and is more in
line with how Linux kernel defines this macro (kernel.h). Can't exactly
lift the kernel implementation because it uses gnu-extension typeof.

Not really a necessary change, but it was causing clang's address
sanitizer to complain.

nsock/src/gh_heap.h

@@ -73,7 +73,7 @@
 
 #if !defined(container_of)
 #define container_of(ptr, type, member) \
-        ((type *)((char *)(ptr)-(char *)(&((type *)0)->member)))
+        ((type *)((char *)(ptr) - offsetof(type, member)))
 #endif