runtime(yaml): Fix flow mapping key detection

fixes: #15196
closes: #15313

Signed-off-by: itchyny <itchyny@cybozu.co.jp>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/syntax/testdir/dumps/yaml_01.dump b/runtime/syntax/testdir/dumps/yaml_01.dump
index ae84e98..bbef58c 100644
--- a/runtime/syntax/testdir/dumps/yaml_01.dump
+++ b/runtime/syntax/testdir/dumps/yaml_01.dump
@@ -11,10 +11,10 @@
 @75
 |p+0#00e0e07&|l|a|i|n| |s|t|r|i|n|g|s|:+0#e000e06&| +0#0000000&@60
 @2|-+0#af5f00255&| +0#0000000&|a| |b| |c| @65
-@2|-+0#af5f00255&| +0#0000000&|a| |*| |b| |&| |c| |@| |d|#| |e| |:| |f| +0#0000e05&|#| |c|o|m@1|e|n|t| +0#0000000&@40
+@2|-+0#af5f00255&| +0#0000000&|a|:|b| |&| |c| |@| |d|#| |e| |*| |f| +0#0000e05&|#| |c|o|m@1|e|n|t| +0#0000000&@42
 @2|-+0#af5f00255&| +0#0000000&|{@1| |f|(|'| |'|)| |}@1| +0#0000e05&|#|8|2|3|4| +0#0000000&@52
 |d+0#00e0e07&|o|u|b|l|e| |q|u|o|t|e|d| |s|t|r|i|n|g|s|:+0#e000e06&| +0#0000000&@52
 @2|-+0#af5f00255&| +0#0000000&|"+0#e000002&@1| +0#0000000&@68
 @2|-+0#af5f00255&| +0#0000000&|"+0#e000002&|a| |b| |c|"|:+0#e000e06&| +0#0000000&|"+0#e000002&|d| |e| |f|"| +0#0000e05&|#| |c|o|m@1|e|n|t| +0#0000000&@44
 @2|-+0#af5f00255&| +0#0000000&|"+0#e000002&|\+0#e000e06&@2|"|\|a|\|b|\|f|\|n|\|r|\|t|\|v|\|0|\|_|\| |\|N|\|L|\|P|\|x|4|1|\|u|0@1|4|1|\|U|0@5|4|1|"+0#e000002&| +0#0000000&@18
-@57|1|9|,|1| @9|1|2|%| 
+@57|1|9|,|1| @9|1@1|%|