Null pointer dereference when parsing ICNS file

reproduce steps:

1. download qt6.3.0 from https://download.qt.io/archive/qt/6.3/6.3.0/single/
2. compile qt with address sanitizer
3. compile image harness from `image.zip`
4. run `./imageharness ./poc`

ASAN output:

```

AddressSanitizer:DEADLYSIGNAL
=================================================================
==3322536==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000028 (pc 0x7f8debed2ca9 bp 0x7ffc22f89490 sp 0x7ffc22f88aa0 T0)
==3322536==The signal is caused by a READ memory access.
==3322536==Hint: address points to the zero page.
#0 0x7f8debed2ca9 in QICNSHandler::read(QImage*) /home/casper/targets/bigproj/qt/aflasan/SRC/qtimageformats/src/plugins/imageformats/icns/qicnshandler.cpp
#1 0x7f8df2113745 in QImageReader::read(QImage*) (/home/casper/targets/bigproj/qt/normal/INSTALL/lib/libQt6Gui.so.6+0x19c745)
#2 0x7f8df2113f48 in QImageReader::read() (/home/casper/targets/bigproj/qt/normal/INSTALL/lib/libQt6Gui.so.6+0x19cf48)
#3 0x4f9d7b in main /home/casper/targets/bigproj/qt/normal/fuzzrun/../../fuzzsrc/image/main.cpp:21:23
#4 0x7f8df13be0b2 in __libc_start_main /build/glibc-sMfBJT/glibc-2.31/csu/../csu/libc-start.c:308:16
#5 0x41c43d in _start (/home/casper/targets/bigproj/qt/normal/fuzzrun/imageharness+0x41c43d)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /home/casper/targets/bigproj/qt/aflasan/SRC/qtimageformats/src/plugins/imageformats/icns/qicnshandler.cpp in QICNSHandler::read(QImage*)
==3322536==ABORTING

```