memory: Check that core is powered on before attempting to use GPU.

- GPU will be released on shutdown, before pages are unmapped.
- On subsequent runs, current_page_table will be not nullptr, but GPU might not be valid yet.

src/core/memory.cpp

@@ -48,7 +48,7 @@ static void MapPages(Common::PageTable& page_table, VAddr base, u64 size, u8* me
               (base + size) * PAGE_SIZE);
 
     // During boot, current_page_table might not be set yet, in which case we need not flush
-    if (current_page_table) {
+    if (Core::System::GetInstance().IsPoweredOn()) {
         Core::System::GetInstance().GPU().FlushAndInvalidateRegion(base << PAGE_BITS,
                                                                    size * PAGE_SIZE);
     }