trace events: fix deadlock from non-recursive mutex.

Release lock over s_pBufferFullCallback:
#4  std::lock_guard<std::mutex>::lock_guard(std::mutex&)
#5  comphelper::TraceEvent::getEventVectorAndClear()
#6  0x00007f2367c61836 in comphelper::TraceEvent::getRecordingAndClear()
#7  0x00007f236877263e in (anonymous namespace)::TraceEventDumper::flushRecordings()
#8  0x00007f2367c60cb7 in comphelper::TraceEvent::addRecording(rtl::OUString const&)

regression from:
commit c2424341ed444647d979a69ae55268e96fad3d56
Date:   Sun Jan 30 10:30:27 2022 +0100
comphelper : use std::mutex in traceevent

Change-Id: Ic89d63d14f06d710937a4da759976ae308c9df45
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/161329
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>