Invalid BPM causes DEBUG_ASSERT in SynControl::setLocalBpm

Encountered with version 2.4-alpha-868-g64a90497fc.

When loading a track using the browse view (i.e. a track not in the library yet), mixxx closed due to a `DEBUG_ASSERT` in `SyncControl::setLocalBpm`:

    debug [Main] SqlTransaction - Started new SQL database transaction on "MIXXX-1"
    DEBUG ASSERT: "isValid()" in function double mixxx::Bpm::value() const at /home/jan/Projects/mixxx/src/track/bpm.h:53
    Aborted (core dumped)

Here's the backtrace:

    >>> bt
    #0 0x00007feaea6b0d22 in raise () at /usr/lib/libc.so.6
    #1 0x00007feaea69a862 in abort () at /usr/lib/libc.so.6
    #2 0x00007feaeace6910 in () at /usr/lib/libQt5Core.so.5
    #3 0x0000558314958a7d in mixxx::(anonymous namespace)::handleMessage(QtMsgType, QMessageLogContext const&, QString const&) (type=<optimized out>, context=<optimized out>, input=<optimized out>) at /home/jan/Projects/mixxx/src/util/logging.cpp:321
    #4 0x00007feaead1b9a0 in () at /usr/lib/libQt5Core.so.5
    #5 0x00007feaead1baba in () at /usr/lib/libQt5Core.so.5
    #6 0x00007feaeace654a in QMessageLogger::critical(char const*, ...) const () at /usr/lib/libQt5Core.so.5
    #7 0x00005583149435a0 in mixxx_debug_assert(char const*, char const*, int, char const*) (assertion=assertion@entry=0x5583154f4ec3 "isValid()", file=file@entry=0x5583154f3e38 "/home/jan/Projects/mixxx/src/track/bpm.h", line=line@entry=53, function=function@entry=0x5583154f3e10 "double mixxx::Bpm::value() const") at /home/jan/Projects/mixxx/src/util/assert.h:9
    #8 0x0000558314993b44 in mixxx_debug_assert_return_true(char const*, char const*, int, char const*) (function=0x5583154f3e10 "double mixxx::Bpm::value() const", line=53, file=0x5583154f3e38 "/home/jan/Projects/mixxx/src/track/bpm.h", assertion=0x5583154f4ec3 "isValid()") at /home/jan/Projects/mixxx/src/util/assert.h:18
    #9 mixxx::Bpm::value() const (this=<synthetic pointer>) at /home/jan/Projects/mixxx/src/track/bpm.h:53
    #10 mixxx::operator*(mixxx::Bpm, double) (multiple=1, bpm=...) at /home/jan/Projects/mixxx/src/track/bpm.h:160
    #11 SyncControl::setLocalBpm(mixxx::Bpm) (this=<optimized out>, localBpm=...) at /home/jan/Projects/mixxx/src/engine/sync/synccontrol.cpp:466
    #12 0x0000558314df1d1f in EngineBuffer::postProcess(int) (this=0x558318011150, iBufferSize=2048) at /home/jan/Projects/mixxx/src/engine/enginebuffer.cpp:1330
    #13 0x0000558314e0102b in EngineMaster::processChannels(int) (this=0x5583170285a0, iBufferSize=<optimized out>) at /home/jan/Projects/mixxx/src/engine/enginemaster.cpp:383
    #14 0x0000558314e01517 in EngineMaster::process(int) (this=0x5583170285a0, iBufferSize=iBufferSize@entry=2048) at /home/jan/Projects/mixxx/src/engine/enginemaster.cpp:410
    #15 0x0000558314af53fb in SoundManager::onDeviceOutputCallback(long) (this=<optimized out>, iFramesPerBuffer=iFramesPerBuffer@entry=1024) at /home/jan/Projects/mixxx/src/soundio/soundmanager.cpp:593
    #16 0x0000558314fb3784 in SoundDevicePortAudio::callbackProcessClkRef(long, float*, float const*, PaStreamCallbackTimeInfo const*, unsigned long) (this=0x55831698d0e0, framesPerBuffer=1024, out=<optimized out>, in=<optimized out>, timeInfo=<optimized out>, statusFlags=<optimized out>) at /home/jan/Projects/mixxx/src/soundio/sounddeviceportaudio.cpp:965
    #17 0x00007feaef5896c4 in () at /usr/lib/libportaudio.so.2
    #18 0x00007feaef58b7c0 in () at /usr/lib/libportaudio.so.2
    #19 0x00007feaef59342b in () at /usr/lib/libportaudio.so.2
    #20 0x00007feaea84b259 in start_thread () at /usr/lib/libpthread.so.0
    #21 0x00007feaea7725e3 in clone () at /usr/lib/libc.so.6

Looks like the new `localBpm` is assumed to be valid, but it isn't, which leads to the assertion when multiplying it with the rate ratio.