bug#25706: 26.0.50; Slow C file fontification

[Ravine Var]

I came across another place where a similar lockup happens
(even with the patches posted here).

https://gitlab.com/wireshark/wireshark/-/raw/master/epan/dissectors/packet-rrc.c

Towards the end of the file, once we get to the function
proto_register_rrc(void), the slowdown of scrolling starts and eventually
things freeze.

Just copying that function to a smaller C file is enough to
reproduce the issue. (I found that C-M-h is a nifty command to do this.)

I can open a new bug report if required.