Add floating patch for ICU 64.2 from unicode-org/icu@18b212f.

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: unicode-org/icu#1155
CVE-ID: CVE-2020-10531
PR-URL: #33572
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Steven R Loomis <srloomis@us.ibm.com>