From Sylvain Munaut via https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=7664 :
packet-gmr1_bcch: Add guards in the SI1/2 choice of segment
Although the CSN1 dissector itself will just stop if there is
no matching segment, it will leave the choice field uninitizalized
and so when we use it to fill some other text, it crashes ...
To protect against that, we put a last choice entry that will always
match. As a bonus, it triggers an explicit error in CSN so you
know something is wrong.