work around bugs in zsh 5.3.1; fixes #651

internal/p10k.zsh

@@ -1575,7 +1575,8 @@ _p9k_custom_prompt() {
   local segment_name=${1:u}
   local command=_POWERLEVEL9K_CUSTOM_${segment_name}
   command=${(P)command}
-  local cmd="${(Q)${(Az)command}[1]}"
+  local parts=("${(@z)command}")
+  local cmd="${(Q)parts[1]}"
   (( $+functions[$cmd] || $+commands[$cmd] )) || return
   local content="$(eval $command)"
   [[ -n $content ]] || return