add check if hypervisor_allocator() exists in the generic_free() method

This fixes crash that occurred if memory was freed/deleted before
the hypervisor_allocator is initialized.

src/hvpp/hvpp/lib/mm.cpp

@@ -126,7 +126,7 @@ namespace detail
 
   void generic_free(void* address) noexcept
   {
-    return mm::hypervisor_allocator()->contains(address)
+    return (mm::hypervisor_allocator() && mm::hypervisor_allocator()->contains(address))
       ? mm::hypervisor_allocator()->free(address)
       : mm::system_allocator()->free(address);
   }