SharedBuffer: Amend commit 2d4d465206

I had the right cause of the SharedBuffer leak, but goofed the fix by
desynching the per-pid refcount and the global refcount.

Fix that, and add a generous sprinkle of asserts to make sure the two
stay in sync.

Fixes #341

(... for real this time)

Kernel/SharedBuffer.h

@@ -32,6 +32,7 @@ public:
 #endif
     }
 
+    void sanity_check(const char* what);
     bool is_shared_with(pid_t peer_pid);
     void* ref_for_process_and_get_address(Process& process);
     void share_with(pid_t peer_pid);