[yaml-frontmatter mode] Fix bug breaking the frontmatter being optional

Issue #3760

mode/yaml-frontmatter/yaml-frontmatter.js

@@ -40,7 +40,7 @@
             state.state = FRONTMATTER
             return yamlMode.token(stream, state.inner)
           } else {
-            stream.state = BODY
+            state.state = BODY
             state.inner = CodeMirror.startState(innerMode)
             return innerMode.token(stream, state.inner)
           }