citra-qt: Add check for valid address to call stack

src/citra_qt/debugger/callstack.cpp

@@ -39,6 +39,9 @@ void CallstackWidget::OnCPUStepped()
         ret_addr = Memory::Read32(addr);
         call_addr = ret_addr - 4; //get call address???
         
+        if (Memory::GetPointer(call_addr) == nullptr)
+            break;
+
         /* TODO (mattvail) clean me, move to debugger interface */
         u32 insn = Memory::Read32(call_addr);
         if (disasm->Decode(insn) == OP_BL)