Prevent silentfile over-read #8563
Fixes negative read-count
```
Thread 45 (Thread 0x7fff9d79c640 (LWP 32440) "butler"):
#0 ?? () from /lib64/libc.so.6
#1 ARDOUR::SilentFileSource::read_unlocked (this=0x5555699f2160, dst=0x7fff9c5d8010, start=60547072, cnt=-47406) at ../libs/ardour/ardour/silentfilesource.h:58
#2 ARDOUR::AudioSource::read (this=0x5555699f2160, dst=0x7fff9c5d8010, start=60547072, cnt=262144) at ../libs/ardour/audiosource.cc:322
#3 ARDOUR::AudioRegion::read_from_sources (this=0x555570859780, srcs=std::vector of length 2, capacity 2 = {...}, limit=33675720, buf=0x7fff9c5d8010, position=60547072, cnt=262144, chan_n=0) at ../libs/ardour/audioregion.cc:747
#4 ARDOUR::AudioRegion::read_at (this=0x555570859780, buf=0x7fff9cdd9010, mixdown_buffer=0x7fff9c5d8010, gain_buffer=0x7fff677ff010, position=60547072, cnt=262144, chan_n=0) at ../libs/ardour/audioregion.cc:587
```
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