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

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

1 files changed, 7 insertions, 4 deletions

diff --git a/src/syntax.c b/src/syntax.c
index 4166ee211c7..df8800e90be 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -2857,10 +2857,13 @@ 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;