Ensure mutexs in avformat-novalidate are initalized

[jlskuz]

For the normal avformat producer this is done in producer_open, which is not called for avfilter-novalidate. However producer_close is called in either case and uses the close mutex. Without this patch the close mutex is not initalized for avformat-novalidate and hence it segfaults.

This is the stack trace:

#  1: QTest::CrashHandler::prepareStackTrace() - 0x00007FFFDFB69130
#  2: UnhandledExceptionFilter() - 0x00007FFFF5011A60
#  3: memcpy() - 0x00007FFFF78F7980
#  4: _C_specific_handler() - 0x00007FFFF78DF7B0
#  5: _chkstk() - 0x00007FFFF78F5D70
#  6: RtlFindCharInUnicodeString() - 0x00007FFFF786DE30
#  7: KiUserExceptionDispatcher() - 0x00007FFFF78F4E70
#  8: pthread_attr_setscope() - 0x00007FFFEDA01528
#  9: producer_avformat_close() - 0x00007FFFDD2CCCC0
# 10: cache_object_close() - 0x00007FFFDD3F78B0
# 11: mlt_cache_purge() - 0x00007FFFDD3F8050
# 12: mlt_service_cache_purge() - 0x00007FFFDD41B360
# 13: producer_close() - 0x00007FFFDD2CD340
# 14: Mlt::Producer::~Producer() - 0x00007FFFE0379AE0
# 15: TestModAvformat::qt_static_metacall() - 0x00007FF77FD01540
# 16: QMetaMethodInvoker::invokeImpl() - 0x00007FFF42BBFB00
# 17: QMetaMethod::invokeImpl() - 0x00007FFF42BBF8F0
# 18: QTest::ignoreMessage() - 0x00007FFFDFB45780
# 19: QTest::ignoreMessage() - 0x00007FFFDFB45780
# 20: QTest::ignoreMessage() - 0x00007FFFDFB45780
# 21: QTest::ignoreMessage() - 0x00007FFFDFB45780
# 22: QTest::qRun() - 0x00007FFFDFB4ADA0
# 23: QTest::qExec() - 0x00007FFFDFB48120
# 24: main() - 0x00007FF77FD01E70
# 25: __scrt_common_main_seh() - 0x00007FF77FD021F4
# 26: BaseThreadInitThunk() - 0x00007FFFF57D2580
# 27: RtlUserThreadStart() - 0x00007FFFF78AAF50

The segfault only happend with MSVC, where we use a different type of pthread implementation, but I belive theoretically the bug exists independent of the plattform.