Handle race in external/breakpad

...that hit once in a UBSan build in UITest_calc_tests4:

> soffice.bin: /data/sbergman/lo-san/core/include/com/sun/star/uno/Reference.h:420: interface_type *com::sun::star::uno::Reference<com::sun::star::xml::crypto::XXMLSecurityContext>::operator->() const [interface_type = com::sun::star::xml::crypto::XXMLSecurityContext]: Assertion `_pInterface != NULL' failed.
> warn:desktop:11041:11425:desktop/source/app/sofficemain.cxx:82: minidump generated: /data/sbergman/lo-san/core/workdir/UITest/calc_tests4/user/crash//0585280b-6f1e-fa99-4b711485-60baa6f1.dmp
> src/client/linux/handler/exception_handler.cc:367:34: runtime error: member call on null pointer of type 'std::vector<google_breakpad::ExceptionHandler *, std::allocator<google_breakpad::ExceptionHandler *> >'
> warn:sal.osl.mutex:11041:11041:sal/osl/unx/mutex.cxx:82: pthread_mutex_destroy failed: Device or resource busy
>  #0 in google_breakpad::ExceptionHandler::SignalHandler(int, siginfo_t*, void*) at <null> (instdir/program/libsofficeapp.so +0x963904)
> warn:unotools.config:11041:11041:unotools/source/config/configmgr.cxx:169: ConfigManager not empty
>  #1  at <null> (/lib64/libpthread.so.0 +0x11fbf)

Change-Id: Icf46adea879c0541a297e5f54f9ce936caa9858b
Reviewed-on: https://gerrit.libreoffice.org/59157
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>