deps: backport ICU-20958 to fix CVE-2020-10531

Add floating patch for ICU 64.2 from https://github.com/unicode-org/icu/commit/18b212f235c8b1dea3b8bbd0accd57340b32ba55.

Original commit message:
    ICU-21032 Backport to 64.x: ICU-20958 Prevent SEGV_MAPERR in append

    See #971

    (cherry picked from commit b7d08bc04a4296982fcef8b6b8a354a9e4e7afca)

Refs: https://unicode-org.atlassian.net/browse/ICU-20958
Refs: https://github.com/unicode-org/icu/pull/1155
CVE-ID: CVE-2020-10531
PR-URL: https://github.com/nodejs/node/pull/33572


Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Steven R Loomis <srloomis@us.ibm.com>