Fix bug in tools/plot-timer-events

tools/plot-timer-events

 #!/bin/sh
 
 # find the name of the log file to process, it must not start with a dash.
 log_file="v8.log"
 for arg in "$@"
 do
   if ! expr "X${arg}" : "^X-" > /dev/null; then
     log_file=${arg}
   fi
 done
 
 tools_path=`cd $(dirname "$0");pwd`
 if test ! "$D8_PATH"; then
   d8_public=`which d8`
   if test -x "$d8_public"; then D8_PATH=$(dirname "$d8_public"); fi
 fi
 
-if test -n "$D8_PATH"; then
+if test ! -n "$D8_PATH"; then
   D8_PATH=$tools_path/..
 fi
 
 d8_exec=$D8_PATH/d8
 
 if test ! -x "$d8_exec"; then
   D8_PATH=`pwd`/out/native
   d8_exec=$D8_PATH/d8
 fi
 
@@ skipping 55 matching lines @@
     -- $@ $options 2>/dev/null > timer-events.plot
 
 success=$?
 if test $success -ne 0; then
     cat timer-events.plot
 else
     cat timer-events.plot | gnuplot > timer-events.png
 fi
 
 rm -f timer-events.plot