_autoenv_debug: fix error with Zsh 5.0.5

> _autoenv_debug:local:1: not valid in this context: handler:

Fixes https://github.com/Tarrasch/zsh-autoenv/issues/38.

autoenv.zsh

@@ -140,7 +140,7 @@ _autoenv_stack_entered_contains() {
 
 # Internal function for debug output. {{{
 _autoenv_debug() {
-  local msg=$1
+  local msg="$1"
   local level=${2:-1}
   if [[ $AUTOENV_DEBUG -lt $level ]]; then
     return