🐛 Fix: undefined variable in reference block (#45)

Co-authored-by: Chris Sewell <chrisj_sewell@hotmail.com>

Author: asmeurer

markdown_it/rules_block/reference.py

@@ -74,6 +74,7 @@ def reference(state: StateBlock, startLine, _endLine, silent):
     string = state.getLines(startLine, nextLine, state.blkIndent, False).strip()
     maximum = len(string)
 
+    labelEnd = None
     pos = 1
     while pos < maximum:
         ch = charCodeAt(string, pos)
@@ -90,7 +91,9 @@ def reference(state: StateBlock, startLine, _endLine, silent):
                 lines += 1
         pos += 1
 
-    if labelEnd < 0 or charCodeAt(string, labelEnd + 1) != 0x3A:  # /* : */
+    if (
+        labelEnd is None or labelEnd < 0 or charCodeAt(string, labelEnd + 1) != 0x3A
+    ):  # /* : */
         return False
 
     # [label]:   destination   'title'