kernel: Nullability check for current_cpu when changing memory page table

This attempts to fix segfault in some tests where page table is set before initializing cpu core (intended behaviour? might be worth a check...)
see: src/tests/core/arm/arm_test_common.cpp
see: src/tests/core/arm/dyncom/arm_dyncom_vfp_tests.cpp

src/core/hle/kernel/kernel.cpp

@@ -53,7 +53,9 @@ void KernelSystem::SetCurrentProcess(std::shared_ptr<Process> process) {
 
 void KernelSystem::SetCurrentMemPageTable(Memory::PageTable* page_table) {
     memory.SetCurrentPageTable(page_table);
+    if (current_cpu != nullptr) {
         current_cpu->PageTableChanged(); // notify the CPU the page table in memory has changed
+    }
 }
 
 void KernelSystem::SetCPU(std::shared_ptr<ARM_Interface> cpu) {