Fix bug #18339 with segfault when $ is typed into empty LaTeX buffer.

Back-ported from trunk revision-id: 2014-08-27T19:40:54Z!eliz@gnu.org

 src/syntax.c (scan_lists): Don't examine positions before BEGV.

2 files changed, 12 insertions, 4 deletions

diff --git a/src/ChangeLog b/src/ChangeLog
index 8ca95025019..b3744eb8b6e 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,8 @@
+2014-08-28  Eli Zaretskii  <eliz@gnu.org>
+
+        * syntax.c (scan_lists): Don't examine positions before BEGV.
+        (Bug#18339)
+
 2014-08-25  Eli Zaretskii  <eliz@gnu.org>
 
         * conf_post.h (_GL_EXECINFO_INLINE) [MSDOS]: Don't define.
diff --git a/src/syntax.c b/src/syntax.c
index 0ee48bb3725..5e697d350ff 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -2859,10 +2859,13 @@ scan_lists (EMACS_INT from, EMACS_INT count, EMACS_INT depth, bool sexpflag)
             case Smath:
               if (!sexpflag)
                 break;
-              temp_pos = dec_bytepos (from_byte);
-              UPDATE_SYNTAX_TABLE_BACKWARD (from - 1);
-              if (from != stop && c == FETCH_CHAR_AS_MULTIBYTE (temp_pos))
-                DEC_BOTH (from, from_byte);
+              if (from > BEGV)
+                {
+                  temp_pos = dec_bytepos (from_byte);
+                  UPDATE_SYNTAX_TABLE_BACKWARD (from - 1);
+                  if (from != stop && c == FETCH_CHAR_AS_MULTIBYTE (temp_pos))
+                    DEC_BOTH (from, from_byte);
+                }
               if (mathexit)
                 {
                   mathexit = 0;