Fix parsing incomplete unicode escape "\uaaa" #779

Job	Run time
Ruby memcheck	56s
ruby-versions / Generate Ruby versions	2s
ubuntu-latest 2.7	1m 39s
ubuntu-latest 3.0	39s
ubuntu-latest 3.1	39s
ubuntu-latest 3.2	33s
ubuntu-latest 3.3	36s
ubuntu-latest 3.4	39s
ubuntu-latest head	1m 27s
ubuntu-latest jruby	1m 7s
ubuntu-latest jruby-head	1m 6s
macos-14 2.7	58s
macos-14 3.0	27s
macos-14 3.1	38s
macos-14 3.2	26s
macos-14 3.3	22s
macos-14 3.4	20s
macos-14 head	25s
macos-14 jruby	1m 6s
macos-14 jruby-head	45s
windows-latest 2.7	5m 5s
windows-latest 3.0	2m 16s
windows-latest 3.1	3m 23s
windows-latest 3.2	3m 8s
windows-latest 3.3	4m 52s
windows-latest 3.4	4m 26s
windows-latest head	2m 23s
ubuntu-24.04-arm 3.4	42s
macos-13 3.4	49s
windows-latest mswin	1m 27s
ubuntu-latest jruby-9.4	1m 5s
macos-latest truffleruby-head	51s
ubuntu-latest truffleruby-head	2m 9s
	47m 26s

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