patch 8.2.4305: tex filetype detection fails

Problem:    Tex filetype detection fails.
Solution:   Check value to be positive. (closes #9704)

src/testdir/test_filetype.vim

@@ -1306,4 +1306,23 @@ func Test_frm_file()
   filetype off
 endfunc
 
+func Test_tex_file()
+  filetype on
+
+  " only tests one case, should do more
+  let lines =<< trim END
+      % This is a sentence.
+
+      This is a sentence.
+  END
+  call writefile(lines, "Xfile.tex")
+  split Xfile.tex
+  call assert_equal('plaintex', &filetype)
+  bwipe
+
+  call delete('Xfile.tex')
+  filetype off
+endfunc
+
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4305,
 /**/
     4304,
 /**/