Fortify RV saving against tempdir being destroyed

Now `rm -rf /var/tmp/bash-rv` would only disable the feature instead of
causing errors to be printed on each prompt.

dotfiles/bash/post.bashrc

 
 __make_ps1d() {
     local rvfile="/var/tmp/bash-rv/$$.lastrv"
-    local lastrv=$(cat $rvfile)
-    if [ $lastrv -gt 0 ];
+    local lastrv
+    read lastrv <<<$(cat $rvfile 2>/dev/null)
+    if [ 0$lastrv -gt 0 ];
     then
         echo "$lastrv"
     else
         echo ""
     fi
-    rm $rvfile
+    rm -f $rvfile
 }
 
 __save_rv() {
-    echo $1 > /var/tmp/bash-rv/$$.lastrv
+    local rvfile="/var/tmp/bash-rv/$$.lastrv"
+    [ -w ${rvfile%/*} ] && echo $1 > $rvfile
 }
 
 make_ps1() {