Fix a rare segfault in syntax.c

* src/syntax.c (Fforward_comment): Prevent the loop for COUNT < 0
from going outside the valid range of character/byte positions.
(Bug#43499)

* doc/lispref/syntax.texi (Syntax Class Table): Mention the
"comment-fence" and "string-fence" as alternative names of 2
syntax classes.

1 files changed, 14 insertions, 11 deletions

diff --git a/src/syntax.c b/src/syntax.c
index a79ab863367..e8b32f5a445 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -2545,20 +2545,23 @@ between them, return t; otherwise return nil.  */)
               bool fence_found = 0;
               ptrdiff_t ini = from, ini_byte = from_byte;
 
-              while (1)
+              if (from > stop)
                 {
-                  DEC_BOTH (from, from_byte);
-                  UPDATE_SYNTAX_TABLE_BACKWARD (from);
-                  c = FETCH_CHAR_AS_MULTIBYTE (from_byte);
-                  if (SYNTAX (c) == Scomment_fence
-                      && !char_quoted (from, from_byte))
+                  while (1)
                     {
-                      fence_found = 1;
-                      break;
+                      DEC_BOTH (from, from_byte);
+                      UPDATE_SYNTAX_TABLE_BACKWARD (from);
+                      c = FETCH_CHAR_AS_MULTIBYTE (from_byte);
+                      if (SYNTAX (c) == Scomment_fence
+                          && !char_quoted (from, from_byte))
+                        {
+                          fence_found = 1;
+                          break;
+                        }
+                      else if (from == stop)
+                        break;
+                      rarely_quit (++quit_count);
                     }
-                  else if (from == stop)
-                    break;
-                  rarely_quit (++quit_count);
                 }
               if (fence_found == 0)
                 {