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Fix crash with hosted clap plugins without MIDI input #1635

Job	Run time
linux-x86_64-debug	1h 13m 35s
linux (aarch64)	25m 30s
headless	37m 22s
linux (armhf)	33m 20s
lto	41m 22s
linux (i386)	23m 32s
macos (universal-10.15)	1h 59m 25s
linux (riscv64)	25m 56s
linux (x86_64)	27m 9s
modaudio (modduo, modduo-static, MODDUO=true)	42m 2s
plugin-validation	1h 32m 27s
modaudio (modduox, modduox-static)	41m 19s
source-tarball	20m 52s
modaudio (moddwarf, moddwarf)	43m 42s
sysdeps	45m 53s
wasm (simd)	1h 12m 35s
windows (win32)	48m 12s
wasm (nosimd)	1h 28m 15s
windows (win64)	41m 45s
	15h 44m 13s

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