Fix IIR filter Q becoming negative #1399

Job	Run time
build (-Denable-profiling=1)	1m 8s
build (-Denable-floats=1 -Denable-profiling=1)	1m 14s
build (-Denable-floats=1)	1m 13s
build (-Denable-trap-on-fpe=1)	1m 10s
build (-Denable-fpe-check=1)	55s
build (-Denable-ipv6=0)	1m 1s
build (-Denable-network=0)	56s
build (-Denable-aufile=0)	1m 0s
build (-DBUILD_SHARED_LIBS=0)	1m 3s
build (-Denable-ubsan=1 -Denable-debug=1)	1m 22s
build (-Denable-debug=1 -DCMAKE_C_FLAGS_DEBUG=-fuse-ld=gold)	1m 20s
build (clang-7, clang++-7)	59s
build (clang-8, clang++-8)	1m 14s
build (clang-10, clang++-10)	1m 3s
build (clang-12, clang++-12)	1m 0s
build (clang-10, clang++-10, -DBUILD_SHARED_LIBS=0)	1m 28s
	18m 6s

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