controller: Prevent crash when db is empty. #3281

Job	Run time
osx clang --disable-ssl	2m 10s
linux rpm fedora	6m 52s
Prepare container	3m 25s
linux gcc --disable-ssl	2m 49s
linux clang --disable-ssl	2m 46s
linux gcc test -300	9m 27s
linux gcc test 301-600	6m 17s
linux gcc test 601- unstable	6m 54s
linux clang test sanitizers -300	12m 29s
linux clang test sanitizers 301-600	9m 7s
linux clang test sanitizers 601- unstable	10m 33s
linux gcc test -ljemalloc -300	9m 58s
linux gcc test -ljemalloc 301-600	6m 35s
linux gcc test -ljemalloc 601- unstable	6m 37s
linux gcc system-test-dpdk dpdk -100	17m 20s
linux gcc system-test-dpdk dpdk 101- unstable	14m 14s
linux gcc system-test-userspace -100	15m 54s
linux gcc system-test-userspace 101- unstable	12m 51s
linux gcc system-test -100	18m 19s
linux gcc system-test 101- unstable	17m 17s
linux clang system-test sanitizers -100	20m 15s
linux clang system-test sanitizers 101- unstable	19m 0s
linux x86 gcc --disable-ssl	2m 38s
	3h 53m 47s

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