Search plugin: fix crash with multiline regexp search

SearchDiskFiles::searchMultiLineRegExp() had two local static variables, it looks like after some recent changes, this causes a crash:

0 QVector::realloc (this=this@entry=0x7ffff0079080 SearchDiskFiles::searchMultiLineRegExp(QFile&)::lineStart, aalloc=, options=..., options@entry=...) at /usr/include/qt5/QtCore/qvector.h:719 1 0x00007ffff005586d in QVector::detach (this=0x7ffff0079080 SearchDiskFiles::searchMultiLineRegExp(QFile&)::lineStart) at /usr/include/qt5/QtCore/qvector.h:410 2 QVector::detach (this=0x7ffff0079080 SearchDiskFiles::searchMultiLineRegExp(QFile&)::lineStart) at /usr/include/qt5/QtCore/qvector.h:402 3 QVector::end (this=0x7ffff0079080 SearchDiskFiles::searchMultiLineRegExp(QFile&)::lineStart) at /usr/include/qt5/QtCore/qvector.h:223 4 QVector::clear (this=0x7ffff0079080 SearchDiskFiles::searchMultiLineRegExp(QFile&)::lineStart) at /usr/include/qt5/QtCore/qvector.h:449 5 QVector::clear (this=0x7ffff0079080 SearchDiskFiles::searchMultiLineRegExp(QFile&)::lineStart) at /usr/include/qt5/QtCore/qvector.h:445 6 SearchDiskFiles::searchMultiLineRegExp (this=this@entry=0x555556fd3150, file=...) at /home/ahmad/kdesrc/applications/kate/addons/search/SearchDiskFiles.cpp:141 7 0x00007ffff0055b1e in SearchDiskFiles::run (this=0x555556fd3150) at /home/ahmad/kdesrc/applications/kate/addons/search/SearchDiskFiles.cpp:51 8 0x00007ffff5bc91f2 in QThreadPoolThread::run (this=0x555556fd33b0) at thread/qthreadpool.cpp:100 9 0x00007ffff5bc5ef1 in QThreadPrivate::start (arg=0x555556fd33b0) at thread/qthread_unix.cpp:329 10 0x00007ffff4bcc299 in start_thread () from /lib64/libpthread.so.0 11 0x00007ffff5686af3 in clone () from /lib64/libc.so.6