Fix slight bug in katex (#21171)

There is a small bug in #20571 whereby `$a a$b b$` will not be correctly
detected as a math inline block of `a a$b b`. This PR fixes this.

Also reenable test cases as per #21340 

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/markup/markdown/meta.go

@@ -88,7 +88,9 @@ func ExtractMetadataBytes(contents []byte, out interface{}) ([]byte, error) {
 		line := contents[start:end]
 		if isYAMLSeparator(line) {
 			front = contents[frontMatterStart:start]
-			body = contents[end+1:]
+			if end+1 < len(contents) {
+				body = contents[end+1:]
+			}
 			break
 		}
 	}