isisd: fix crash when changing isis type (backport #20171)

[mergify]

The following crash happens, when moving from level-2 to level-1 an isis flex-algorithm configuration

warning: 44 ./nptl/pthread_kill.c: No such file or directory
[Current thread is 1 (Thread 0x7108d4cb2980 (LWP 1023))]
(gdb) bt
#0 __pthread_kill_implementation (no_tid=0, signo=11,
threadid=) at ./nptl/pthread_kill.c:44
#1 __pthread_kill_internal (signo=11, threadid=)
at ./nptl/pthread_kill.c:78
#2 __GI___pthread_kill (threadid=, signo=signo@entry=11)
at ./nptl/pthread_kill.c:89
#3 0x00007108d3e4527e in __GI_raise (sig=11) at ../sysdeps/posix/raise.c:26
#4 0x00007108d4b44926 in core_handler (signo=11, siginfo=0x7ffe7c10fb30,
context=0x7ffe7c10fa00)
at /build/make-pkg/output/_packages/cp-routing/src/lib/sigevent.c:248
#5
#6 0x00005b5d803bf07b in isis_spf_invalidate_routes (tree=0x0)
at /build/make-pkg/output/_packages/cp-routing/src/isisd/isis_spf.c:2118
#7 0x00005b5d803fb23e in isis_area_invalidate_routes (area=0x5b5db8d5be40,
levels=1)
at /build/make-pkg/output/_packages/cp-routing/src/isisd/isisd.c:3152
#8 0x00005b5d803bf280 in isis_run_spf_cb (thread=0x7ffe7c110180)
at /build/make-pkg/output/_packages/cp-routing/src/isisd/isis_spf.c:2165
#9 0x00007108d4b5ff7f in event_call (thread=0x7ffe7c110180)
at /build/make-pkg/output/_packages/cp-routing/src/lib/event.c:2011
#10 0x00007108d4adb761 in frr_run (master=0x5b5db7f7ca40)
at /build/make-pkg/output/_packages/cp-routing/src/lib/libfrr.c:1219
#11 0x00005b5d8038333a in main (argc=5, argv=0x7ffe7c1103d8,
--Type for more, q to quit, c to continue without paging--
envp=0x7ffe7c110408)
at /build/make-pkg/output/_packages/cp-routing/src/isisd/isis_main.c:360
(gdb)

Fix this by adding protection before invalidating all routes.

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This is an automatic backport of pull request #20171 done by Mergify.