ICU-21032 Backport to 64 of ICU-20958

[srl295]

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

See #971

(cherry picked from commit b7d08bc)

[srl295]

/azp run CI-Exhaustive

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[srl295]

@jefgen on the CI-J (Which isn't relevant for this)

##[error]The job running on agent Hosted Agent ran longer than the maximum time of 20 minutes. For more information, see https://go.microsoft.com/fwlink/?linkid=2077134

[jefgen]

@jefgen on the CI-J (Which isn't relevant for this)

##[error]The job running on agent Hosted Agent ran longer than the maximum time of 20 minutes. For more information, see https://go.microsoft.com/fwlink/?linkid=2077134

It looks like both Azure Pipelines and Travis have the same error/issue with the ICU4J builds:

[ivy:retrieve] :::: ERRORS
[ivy:retrieve] 	SERVER ERROR: HTTPS Required url=http://repo1.maven.org/maven2/junit/junit/4.12/junit-4.12.pom
[ivy:retrieve] 	SERVER ERROR: HTTPS Required url=http://repo1.maven.org/maven2/junit/junit/4.12/junit-4.12.jar
[ivy:retrieve] 	SERVER ERROR: HTTPS Required url=http://repo1.maven.org/maven2/pl/pragmatists/JUnitParams/1.0.5/JUnitParams-1.0.5.pom
[ivy:retrieve] 	SERVER ERROR: HTTPS Required url=http://repo1.maven.org/maven2/pl/pragmatists/JUnitParams/1.0.5/JUnitParams-1.0.5.jar
[ivy:retrieve] 

IIRC, this is actually ICU-20942, which I fixed with #965 (in 66.1).

I suppose if we wanted to get the ICU4J CI builds working in the maint/maint-66 branch then we could cherry-pick that into 64.1 too, but since this change is only for ICU4C, perhaps it's not worth it.

[srl295]

OK, probably should backport 20942 at some point… good to know thanks