...so that e.g. main thread in SalYieldMutex::doAcquire could reset
m_aInMainTHreadCondition and then block waiting on it only after another thread
had set it.  (Saw such a deadlock in some 'make check' CppunitTest.)

Change-Id: I5c676956a2bec6bf8f94d7dbeee64f100db39bd3
Reviewed-on: https://gerrit.libreoffice.org/44501Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>