Fix crash on startup when there are no MIDI devices #1319

Job	Run time
build (-Denable-profiling=1)	1m 55s
build (-Denable-floats=1 -Denable-profiling=1)	1m 44s
build (-Denable-floats=1)	54s
build (-Denable-trap-on-fpe=1)	57s
build (-Denable-fpe-check=1)	1m 14s
build (-Denable-ipv6=0)	56s
build (-Denable-network=0)	1m 18s
build (-Denable-aufile=0)	1m 17s
build (-DBUILD_SHARED_LIBS=0)	1m 7s
build (-Denable-ubsan=1 -Denable-debug=1)	1m 15s
build (-Denable-debug=1 -DCMAKE_C_FLAGS_DEBUG=-fuse-ld=gold)	1m 20s
build (clang-7, clang++-7)	55s
build (clang-8, clang++-8)	55s
build (clang-10, clang++-10)	1m 9s
build (clang-12, clang++-12)	1m 2s
build (clang-10, clang++-10, -DBUILD_SHARED_LIBS=0)	1m 7s
	19m 5s

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