Check for tmux session in prompt

Before invoking the `tmux` command to unset the vim pane variable first
check that a `tmux` session exists.

prompt_fresh_setup

@@ -110,9 +110,9 @@ fresh_line_one() {
   fi
 
   # Unset vim/tmux navigate flag to handle C-z and multiple vim jobs.
-  if [[ ! -z "$TMUX" ]]; then
-    tmux set-window-option -u @vim$TMUX_PANE
-  fi
+  [[ ! -z "$TMUX" ]] && \
+    [[ "`tmux show-window-options`" = *"@vim$TMUX_PANE"* ]] && \
+      tmux set-window-option -u @vim$TMUX_PANE
 
   # If a virtualenv is enabled, display it's basename
   if [[ ! -z "$VIRTUAL_ENV" ]]; then