out_gelf: fix crash after missing short_message key #11179

Job	Run time
run-ubuntu-unit-tests (-DFLB_JEMALLOC=On, gcc)	14m 44s
run-ubuntu-unit-tests (-DFLB_JEMALLOC=On, clang)	14m 44s
run-ubuntu-unit-tests (-DFLB_JEMALLOC=Off, gcc)	14m 0s
run-ubuntu-unit-tests (-DFLB_JEMALLOC=Off, clang)	14m 56s
run-ubuntu-unit-tests (-DFLB_SMALL=On, gcc)	13m 56s
run-ubuntu-unit-tests (-DFLB_SMALL=On, clang)	14m 48s
run-ubuntu-unit-tests (-DSANITIZE_ADDRESS=On, gcc)	14m 18s
run-ubuntu-unit-tests (-DSANITIZE_ADDRESS=On, clang)	14m 16s
run-ubuntu-unit-tests (-DSANITIZE_UNDEFINED=On, gcc)	14m 14s
run-ubuntu-unit-tests (-DSANITIZE_UNDEFINED=On, clang)	13m 59s
run-ubuntu-unit-tests (-DFLB_COVERAGE=On, gcc)	14m 29s
run-ubuntu-unit-tests (-DFLB_SANITIZE_MEMORY=On, gcc)	13m 48s
run-ubuntu-unit-tests (-DFLB_SANITIZE_MEMORY=On, clang)	13m 58s
run-ubuntu-unit-tests (-DFLB_SANITIZE_THREAD=On, gcc)	14m 2s
run-ubuntu-unit-tests (-DFLB_SANITIZE_THREAD=On, clang)	14m 1s
run-ubuntu-unit-tests (-DFLB_SIMD=On, gcc)	14m 8s
run-ubuntu-unit-tests (-DFLB_SIMD=On, clang)	13m 51s
run-ubuntu-unit-tests (-DFLB_SIMD=Off, gcc)	14m 4s
run-ubuntu-unit-tests (-DFLB_SIMD=Off, clang)	13m 39s
run-macos-unit-tests	0s
run-aarch64-unit-tests	0s
run-qemu-ubuntu-unit-tests	0s
Unit tests (matrix)	1s
	4h 29m 56s

Provide feedback