kfazi authored Jul 22, 2019 and Brad King committed Jul 23, 2019

In commit fec441ec (Teach CROSSCOMPILING_EMULATOR to support
arguments, 2019-05-30, v3.15.0-rc1~6^2) the new member
`cmCustomCommandGenerator::EmulatorsWithArguments` was not initialized
to the proper size.  Fix this and add a test case covering the crash
that could occur with multiple commands where an emulator appears only
in a later command.

Fixes: #19500
Co-Author: Brad King <brad.king@kitware.com>