Kernel: Allow passing null pointer to delete

The C++ standard says that it's legal to call the `delete` operator with
a null pointer argument, in which case it should be a no-op. I
encountered this issue when running a kernel that's compiled with Clang.
I assume this fact was used for some kind of optimization.

Kernel/Heap/SlabAllocator.h

@@ -33,6 +33,8 @@ public:                                                                      \
     }                                                                        \
     void operator delete(void* ptr) noexcept                                 \
     {                                                                        \
+        if (!ptr)                                                            \
+            return;                                                          \
         slab_dealloc(ptr, sizeof(type));                                     \
     }                                                                        \
                                                                              \