Using `faustcompile` and `fluidEngine` causes crash

[nwhetsell]

With Csound 6.18.1, FluidSynth 2.4.2, and Faust 2.77.3, running

cat <<EOS > test.orc
gi_programHandle faustcompile "process = _;", "--vectorize --loop-variant 1"
gi_fluidEngineNumber fluidEngine
scoreline_i "e"
EOS
csound --orc --output=dac test.orc

results in an uncaught exception:

libc++abi: terminating due to uncaught exception of type std::__1::system_error: recursive_mutex lock failed: Invalid argument

Moving faustcompile into an instrument—

cat <<EOS > test.orc
gi_fluidEngineNumber fluidEngine
instr 1
    i_programHandle faustcompile "process = _;", "--vectorize --loop-variant 1"
endin
scoreline_i {{
i 1 0 1
e
}}
EOS
csound --orc --output=dac test.orc

—produces a segfault:

csound command: Segmentation fault

This issue began to occur with Faust 2.77.3 (see Homebrew/homebrew-core#202599).