add guard clause for alphabet size (#3897) #13403

Job	Run time
Ubuntu Build release	11m 33s
Ubuntu Build debug	9m 42s
Ubuntu Build asan	41m 45s
Ubuntu Build msan	1h 8m 16s
Ubuntu Build tsan	56m 34s
Ubuntu Build scalar	11m 49s
Ubuntu Build lowprecision	11m 16s
Ubuntu Build coverage	2h 27m 55s
Ubuntu Build release-nojpeg	9m 16s
Ubuntu Build release-lcms2	8m 58s
Ubuntu Build release-system-lcms2	8m 38s
Ubuntu Build release:minimal	8m 39s
Ubuntu Build release:gcc8	9m 11s
Ubuntu Build release:clang-7	9m 41s
MacOS Build release:osx	6m 41s
	6h 59m 54s

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