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.

2 files changed, 24 insertions, 19 deletions

diff --git a/doc/lispref/syntax.texi b/doc/lispref/syntax.texi
index b3c77665bae..b99b5de0b31 100644
--- a/doc/lispref/syntax.texi
+++ b/doc/lispref/syntax.texi
@@ -256,10 +256,11 @@ look in the standard syntax table to find the syntax of this
 character.
 
 @item Generic comment delimiters: @samp{!}
-Characters that start or end a special kind of comment.  @emph{Any}
-generic comment delimiter matches @emph{any} generic comment
-delimiter, but they cannot match a comment starter or comment ender;
-generic comment delimiters can only match each other.
+(This syntax class is also known as ``comment-fence''.)  Characters
+that start or end a special kind of comment.  @emph{Any} generic
+comment delimiter matches @emph{any} generic comment delimiter, but
+they cannot match a comment starter or comment ender; generic comment
+delimiters can only match each other.
 
 This syntax class is primarily meant for use with the
 @code{syntax-table} text property (@pxref{Syntax Properties}).  You
@@ -268,10 +269,11 @@ first and last characters of the range @code{syntax-table} properties
 identifying them as generic comment delimiters.
 
 @item Generic string delimiters: @samp{|}
-Characters that start or end a string.  This class differs from the
-string quote class in that @emph{any} generic string delimiter can
-match any other generic string delimiter; but they do not match
-ordinary string quote characters.
+(This syntax class is also known as ``string-fence''.)  Characters
+that start or end a string.  This class differs from the string quote
+class in that @emph{any} generic string delimiter can match any other
+generic string delimiter; but they do not match ordinary string quote
+characters.
 
 This syntax class is primarily meant for use with the
 @code{syntax-table} text property (@pxref{Syntax Properties}).  You
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)
                 {