AMQ-8226 match any amount of spaces before activemq PID in bin/activemq startup script

(cherry picked from commit f1190f8feddb68e27eb1b7484f2a0928e32996f9)
diff --git a/assembly/src/release/bin/activemq b/assembly/src/release/bin/activemq
index dbd641e..7f72826 100755
--- a/assembly/src/release/bin/activemq
+++ b/assembly/src/release/bin/activemq
@@ -387,7 +387,7 @@
         return 2
        fi
        local activemq_pid="`cat "$pidfile"`"
-       local RET="`ps -o "pid,args" | grep "^$activemq_pid\s.*java"`"
+       local RET="`ps -o "pid,args" | grep "^\s*$activemq_pid\s.*java"`"
        if [ -n "$RET" ];then
          return 0;
        else