'main': Prevent an infinite loop when $PWD had been deleted

Fixes #819

highlighters/main/main-highlighter.zsh

@@ -1240,9 +1240,14 @@ _zsh_highlight_main_highlighter_check_path()
   fi
   tmp_path=$tmp_path:a
 
+  local candidate_new_tmp_path
   while [[ $tmp_path != / ]]; do
     [[ -n ${(M)ZSH_HIGHLIGHT_DIRS_BLACKLIST:#$tmp_path} ]] && return 1
-    tmp_path=$tmp_path:h
+    candidate_new_tmp_path=${tmp_path:h}
+    if [[ $candidate_new_tmp_path == $tmp_path ]]; then
+      break
+    fi
+    tmp_path=${candidate_new_tmp_path}
   done
 
   if (( in_command_position )); then