__run_duration returns 0 when it can't calculate the command run duration...

__run_duration returns 0 when it can't calculate the command run duration (when the shell doesn't have support for PS0, like ksh).

.bashrc

@@ -211,12 +211,16 @@ match_ssl_pair () {
 }
 
 __run_duration () {
-    # Assumes that $HOME/.prerun.$$ exists.
+    if [ -f "$HOME/.prerun.$$" ]
+    then
         local endtime starttime
         endtime="$(date +%s)"
         starttime="$(cat "$HOME/.prerun.$$")"
         rm "$HOME/.prerun.$$"
         echo "$(( endtime - starttime ))"
+    else
+        echo 0
+    fi
 }
 
 __prerun () {