Check result from c-backward-token-2 to avoid infinite loop

This fixes bug #33784.

* lisp/progmodes/cc-fonts.el (c-get-fontification-context): While moving back
over enclosing parentheses, check that c-backward-token-2 actually moves.
1 job for emacs-26 in 2 seconds (queued for 1 second)
Status Job ID Name Coverage
failed #343


Name Stage Failure
test Test There has been a runner system failure, please try again
Running with gitlab-runner 11.4.2 (cf91d5e1)
on gitlab-runner-docker cff9d373
Using Docker executor with image debian:stretch ...
Pulling docker image debian:stretch ...
Using docker image sha256:4879790bd60d439cfe39c063660eef7af525d5f6f1cbb701a14c7cfc11cbfcf7 for debian:stretch ...
ERROR: Job failed (system failure): Error response from daemon: open /var/run/docker/libcontainerd/containerd/b27b06a84372a064bfd804b54f48e72546bb605da8fe0f7bb98b133643b8c652/init/shim-log.json: no such file or directory (executor_docker.go:1015:0s)