in_kafka: Fix in kafka json parsing #7625

Job	Run time
run-ubuntu-unit-tests (-DFLB_JEMALLOC=On, gcc)	20m 3s
run-ubuntu-unit-tests (-DFLB_JEMALLOC=On, clang)	18m 26s
run-ubuntu-unit-tests (-DFLB_JEMALLOC=Off, gcc)	17m 43s
run-ubuntu-unit-tests (-DFLB_JEMALLOC=Off, clang)	22m 10s
run-ubuntu-unit-tests (-DFLB_SMALL=On, gcc)	18m 47s
run-ubuntu-unit-tests (-DFLB_SMALL=On, clang)	18m 33s
run-ubuntu-unit-tests (-DSANITIZE_ADDRESS=On, gcc)	18m 10s
run-ubuntu-unit-tests (-DSANITIZE_ADDRESS=On, clang)	18m 40s
run-ubuntu-unit-tests (-DSANITIZE_UNDEFINED=On, gcc)	18m 21s
run-ubuntu-unit-tests (-DSANITIZE_UNDEFINED=On, clang)	19m 17s
run-ubuntu-unit-tests (-DFLB_COVERAGE=On, gcc)	49m 52s
run-ubuntu-unit-tests (-DFLB_SANITIZE_MEMORY=On, gcc)	17m 54s
run-ubuntu-unit-tests (-DFLB_SANITIZE_MEMORY=On, clang)	17m 12s
run-ubuntu-unit-tests (-DFLB_SANITIZE_THREAD=On, gcc)	18m 51s
run-ubuntu-unit-tests (-DFLB_SANITIZE_THREAD=On, clang)	1h 0m 14s
run-macos-unit-tests	0s
Unit tests (matrix)	1s
	5h 54m 14s

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