fix(_comp_command_offset): work around nounset

Author: akinomyoga

bash_completion

@@ -2860,7 +2860,7 @@ _comp_command_offset()
         _comp_compgen_commands
     else
         _comp_dequote "${COMP_WORDS[0]}" || REPLY=${COMP_WORDS[0]}
-        local cmd=$REPLY compcmd=$REPLY
+        local cmd=${REPLY-} compcmd=${REPLY-}
         local cspec=$(complete -p -- "$cmd" 2>/dev/null)
 
         # If we have no completion for $cmd yet, see if we have for basename