mirror.c: Fix ovn-controller crash when mirror port is deleted from ovs. #2008

Job	Run time
osx clang --disable-ssl	6m 33s
linux gcc --disable-ssl	3m 25s
linux clang --disable-ssl	3m 30s
linux gcc test -500	27m 47s
linux gcc test 501-1000	14m 0s
linux gcc test 1001-	7m 43s
linux clang test sanitizers -300	14m 34s
linux clang test sanitizers 301-600	26m 45s
linux clang test sanitizers 601-900	12m 6s
linux clang test sanitizers 901-1200	13m 16s
linux clang test sanitizers 1201-	7m 18s
linux gcc test -ljemalloc -500	24m 59s
linux gcc test -ljemalloc 501-1000	15m 32s
linux gcc test -ljemalloc 1001-	7m 45s
linux clang test -ljemalloc -500	31m 31s
linux clang test -ljemalloc 501-1000	12m 55s
linux clang test -ljemalloc 1001-	8m 36s
linux gcc system-test -100	15m 40s
linux gcc system-test 101-200	22m 3s
linux gcc system-test 201-	17m 45s
linux clang system-test sanitizers -100	19m 48s
linux clang system-test sanitizers 101-200	24m 0s
linux clang system-test sanitizers 201-	19m 26s
linux gcc m32 --disable-ssl	3m 33s
	6h 0m 30s

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