fix _get_icon on ZSH 5.1

ZSH 5.1 has crippled ${(P)...}. For example, the following code prints "broken":

    unset foo
    echo ${${(P)foo}+broken}

In ZSH 5.2 and later this code print an empty line.

Fixes #91.

internal/p10k.zsh

@@ -77,8 +77,9 @@ _p9k_cache_get() {
 
 # Sets _P9K_RETVAL to the icon whose name is supplied via $1.
 _p9k_get_icon() {
-  local var_name=POWERLEVEL9K_$1
-  _P9K_RETVAL=${(g::)${${(P)var_name}-$icons[$1]}}
+  local x=POWERLEVEL9K_$1
+  (( $+parameters[$x] )) && x=${(P)x} || x=$icons[$1]
+  _P9K_RETVAL=${(g::)x}
   [[ $_P9K_RETVAL != $'\b'? ]] || _P9K_RETVAL="%{$_P9K_RETVAL%}"  # penance for past sins
 }