Merge 585696eeeb into c4d9559184

To reproduce:

    % mkdir /tmp/foo
    % cd /tmp/foo
    % rmdir /tmp/foo
    % exec zsh -f
    % source ~/zsh-syntax-highlighting/zsh-syntax-highlighting.zsh

Press 'a' and zle will hang.

highlighters/main/main-highlighter.zsh

@@ -354,6 +354,7 @@ _zsh_highlight_highlighter_main_paint()
     'pkexec' '' # doesn't take short options; immune to #121 because it's usually not passed --option flags
     # Not listed: -h, which has two different meanings.
     'sudo' Cgprtu:AEHPSbilns:eKkVv # as of sudo 1.8.21p2
+    'run0' ugD:h # systemd 256.6
     'stdbuf' ioe:
     'eatmydata' ''
     'catchsegv' ''
@@ -1235,14 +1236,18 @@ _zsh_highlight_main_highlighter_check_path()
   if [[ $expanded_path[1] == / ]]; then
     tmp_path=$expanded_path
   else
-    tmp_path=$PWD/$expanded_path
+    # Unlike $PWD, ${(%):-%/} predictably expands to '.' if the current
+    # working directory doesn't exist.
+    tmp_path=${(%):-%/}/$expanded_path
   fi
   tmp_path=$tmp_path:a
 
+  if [[ $tmp_path == /* ]]; then
     while [[ $tmp_path != / ]]; do
       [[ -n ${(M)ZSH_HIGHLIGHT_DIRS_BLACKLIST:#$tmp_path} ]] && return 1
       tmp_path=$tmp_path:h
     done
+  fi
 
   if (( in_command_position )); then
     if [[ -x $expanded_path ]]; then