patch 9.1.1072: 'diffopt' "linematch" cannot be used with {n} less than 10

Problem:  'diffopt' "linematch" cannot be used with {n} less than 10
          digits (after v9.1.1022)
Solution: Fix off-by-one error when checking for digit (zeertzjq)

closes: #16577

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/gen_opt_test.vim b/src/testdir/gen_opt_test.vim
index 30de286..2dfb070 100644
--- a/src/testdir/gen_opt_test.vim
+++ b/src/testdir/gen_opt_test.vim
@@ -178,9 +178,11 @@
       \		'icase', 'iwhite', 'iwhiteall', 'horizontal', 'vertical',
       \		'closeoff', 'hiddenoff', 'foldcolumn:0', 'foldcolumn:12',
       \		'followwrap', 'internal', 'indent-heuristic', 'algorithm:myers',
-      \		'algorithm:minimal', 'algorithm:patience',
-      \		'algorithm:histogram', 'icase,iwhite'],
-      \		['xxx', 'foldcolumn:xxx', 'algorithm:xxx', 'algorithm:']],
+      \		'icase,iwhite', 'algorithm:minimal', 'algorithm:patience',
+      \		'algorithm:histogram', 'linematch:5'],
+      \		['xxx', 'foldcolumn:', 'foldcolumn:x', 'foldcolumn:xxx',
+      \		'linematch:', 'linematch:x', 'linematch:xxx', 'algorithm:',
+      \		'algorithm:xxx', 'context:', 'context:x', 'context:xxx']],
       \ 'display': [['', 'lastline', 'truncate', 'uhex', 'lastline,uhex'],
       \		['xxx']],
       \ 'eadirection': [['', 'both', 'ver', 'hor'], ['xxx', 'ver,hor']],