[LyX/master] fixup convert_lineno (#13036)

[Juergen Spitzmueller]

commit b00ea5116678fcf7ebcc5c46ef76b9501d829f7c
Author: Juergen Spitzmueller <spitz at lyx.org>
Date:   Fri Jan 26 10:30:31 2024 +0100

    fixup convert_lineno (#13036)
    
    This routine assumed \linenumbers to be preceded by \usepackage{lineno}
    and that by a lyx2lyx comment.
    
    Of course, this is not the case if users loaded lineno manually.
---
 lib/lyx2lyx/lyx_2_4.py |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/lib/lyx2lyx/lyx_2_4.py b/lib/lyx2lyx/lyx_2_4.py
index 16d6a99..687c3fb 100644
--- a/lib/lyx2lyx/lyx_2_4.py
+++ b/lib/lyx2lyx/lyx_2_4.py
@@ -1915,13 +1915,14 @@ def convert_lineno(document):
     use_lineno = 0
     options = ""
     i = find_token(document.preamble, "\\linenumbers", 1)
-    if i > -1:
+    if i > 0:
         usepkg = re.match(r"\\usepackage(.*){lineno}", document.preamble[i-1])
         if usepkg:
             use_lineno = 1
             options = usepkg.group(1).strip("[]")
             del(document.preamble[i-1:i+1])
-            del_token(document.preamble, "% Added by lyx2lyx", i-2, i-1)
+            if i > 1:
+                del_token(document.preamble, "% Added by lyx2lyx", i-2, i-1)
 
     k = find_token(document.header, "\\index ")
     if options == "":