ICU-23305 Fix int64_t overflow in doParse

[FrankYFTang]

Tested by fuzzer. The test string is difficult to extract as

od -x ~/Downloads/*61056
0000000 2db7 003e 0020 0000 0000 0000 0000 0000
0000020 0000 0000 0000 0000 0000 0000 0000 0000
*
0000140 0000 0000 0001 0000 bc03 3b00 3c00 3000
0000160 003b 0078 0030 0078 003a 003e 0025 403e
0000200 003e 3e00 3eff 251a 3e00 3e40 3e00 703e
0000220 756c 006e 7367 6953 003b 0025 403e 003a
0000240 893e 3b00 1000 0000 0000 0000 1000 3c3e
0000260 003b 003e 003b 0002 0034 003c 003b 003c
0000300 003c 003c 3a40 3e00 0089 003b 0010 0000
0000320 0000 0000 3e10 3b3c 3e00 3b00 0200 3400
0000340 3c00 3b00 3c00 3c00 3c00 0000 0000 0300
0000360 0000 003c 003c 003c 0000 0000 0003 003e
0000400 003b 0000 0000 003b 001f 403e 007a 003e
0000420 003b 0000 1000 3c3e 003b 003e 003b 0002
0000440 0034 003c 003b 003c 003c 003c 0059 0000
0000460 0000 0000 1616 3431 0000 0000

Checklist

• Required: Issue filed: ICU-23305
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-23305 Fix xyz"
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-23305 Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable
• Approver: Feel free to merge on my behalf

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Notice: the branch changed across the force-push!

• icu4c/source/i18n/nfsubs.cpp is different

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