Prevent crash when saving RGBA image where only one channel was added… #2344

Job	Run time
Linux.1: Release / steps	13m 36s
Linux.2: Debug / steps	24m 16s
Linux.3: static / steps	13m 59s
Linux.4: threads=OFF / steps	12m 42s
Linux.5: pkgconfig=OFF, examples=OFF, tools=OFF, docs=OFF, internal deflate / steps	4m 6s
Linux.6: custom namespace / steps	13m 38s
Linux.7: clang / steps	13m 51s
Linux.8: vfx2023 / steps	13m 30s
Windows.2: Debug / steps	39m 29s
Windows.1: Release / steps	13m 26s
Windows.3: static / steps	12m 14s
Windows.4: threads=OFF / steps	12m 32s
Windows.5: pkgconfig=OFF, examples=OFF, tools=OFF, docs=OFF, internal imath+deflate / steps	2m 2s
Windows.6: vfx2022 / steps	13m 31s
macOS.1: Release / steps	8m 29s
macOS.2: Debug / steps	19m 46s
macOS.3: static / steps	7m 43s
macOS.4: threads=OFF / steps	7m 6s
macOS.5: pkgconfig=OFF, examples=OFF, tools=OFF, docs=OFF, internal imath+deflate / steps	1m 21s
macOS.6: macos-13 / steps	19m 39s
	4h 26m 56s

Provide feedback