filter_lua: add index to arraylength call #8232

Job	Run time
run-ubuntu-unit-tests (-DFLB_JEMALLOC=On, gcc)	19m 35s
run-ubuntu-unit-tests (-DFLB_JEMALLOC=On, clang)	19m 40s
run-ubuntu-unit-tests (-DFLB_JEMALLOC=Off, gcc)	18m 56s
run-ubuntu-unit-tests (-DFLB_JEMALLOC=Off, clang)	17m 44s
run-ubuntu-unit-tests (-DFLB_SMALL=On, gcc)	19m 13s
run-ubuntu-unit-tests (-DFLB_SMALL=On, clang)	19m 5s
run-ubuntu-unit-tests (-DSANITIZE_ADDRESS=On, gcc)	18m 51s
run-ubuntu-unit-tests (-DSANITIZE_ADDRESS=On, clang)	18m 17s
run-ubuntu-unit-tests (-DSANITIZE_UNDEFINED=On, gcc)	18m 46s
run-ubuntu-unit-tests (-DSANITIZE_UNDEFINED=On, clang)	18m 51s
run-ubuntu-unit-tests (-DFLB_COVERAGE=On, gcc)	51m 4s
run-ubuntu-unit-tests (-DFLB_SANITIZE_MEMORY=On, gcc)	18m 50s
run-ubuntu-unit-tests (-DFLB_SANITIZE_MEMORY=On, clang)	18m 48s
run-ubuntu-unit-tests (-DFLB_SANITIZE_THREAD=On, gcc)	19m 4s
run-ubuntu-unit-tests (-DFLB_SANITIZE_THREAD=On, clang)	18m 19s
run-macos-unit-tests (-DFLB_JEMALLOC=Off)	11m 3s
run-macos-unit-tests (-DFLB_SANITIZE_MEMORY=On)	14m 14s
run-macos-unit-tests (-DFLB_SANITIZE_THREAD=On)	11m 48s
Unit tests (matrix)	1s
	5h 52m 9s

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