use-of-uninitialized values in print-isakmp.c

[mcr]

need to fix print-isakmp.c. Test case:

$ CC=clang CFLAGS='-fno-omit-frame-pointer -fsanitize=memory -fno-optimize-sibling-calls -fPIC -g3 -ggdb3 -O0' ./configure

$ make -s clean all

$ ./tcpdump -#nvvvv -E "file tests/ikev2pI2-secrets.txt" -r tests/ikev2pI2.pcap 2>&1|m

1) tcpdump-4.9 branch:

reading from file tests/ikev2pI2.pcap, link-type NULL (BSD loopback)
==135139==WARNING: MemorySanitizer: use-of-uninitialized-value
#0 0x6960c6 in ikev2_ID_print 
...

[fxlb]

Workaround commited.