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

@@ -1915,12 +1915,13 @@ 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])
+            if i > 1:
                 del_token(document.preamble, "% Added by lyx2lyx", i-2, i-1)
 
     k = find_token(document.header, "\\index ")