CC Mode: Fix infinite loop in c-brace-stack-at.

This fixes bug #59038.  The infinite loop occurred with a buffer containing
Base-64 encoded text rather than appropriate C Mode text.

* lisp/progmodes/cc-engine.el (c-update-brace-stack): Don't execute
c-beginning-of-current-token unless the previous c-syntactic-re-search-forward
has succeeded at least once.

1 files changed, 4 insertions, 3 deletions

diff --git a/lisp/progmodes/cc-engine.el b/lisp/progmodes/cc-engine.el
index d730fddeb08..8cf882d5699 100644
--- a/lisp/progmodes/cc-engine.el
+++ b/lisp/progmodes/cc-engine.el
@@ -6158,9 +6158,10 @@ comment at the start of cc-engine.el for more info."
             (setq s (cdr s))))
          ((c-keyword-member kwd-sym 'c-flat-decl-block-kwds)
           (push 0 s))))
-      ;; The failing `c-syntactic-re-search-forward' may have left us in the
-      ;; middle of a token, which might be a significant token.  Fix this!
-      (c-beginning-of-current-token)
+      (when (> prev-match-pos 1)      ; Has the search matched at least once?
+        ;; The failing `c-syntactic-re-search-forward' may have left us in the
+        ;; middle of a token, which might be a significant token.  Fix this!
+        (c-beginning-of-current-token))
       (cons (point)
             (cons bound-<> s)))))