fix(mutt): default mail dir assignment

Author: scop

completions/mutt

@@ -134,7 +134,7 @@ _muttfiledir()
     muttrc=$(_muttrc)
     if [[ $cur == [=+]* ]]; then
         folder="$("$muttcmd" -F "$muttrc" -Q folder 2>/dev/null | command sed -e 's|^folder=\"\(.*\)\"$|\1|')"
-        : folder:=~/Mail
+        [[ $folder ]] || folder=~/Mail
 
         # Match any file in $folder beginning with $cur
         # (minus the leading '=' sign).