vboxwrapper: fix compiler warning

svn path=/trunk/boinc/; revision=24219

checkin_notes

@@ -6092,6 +6092,10 @@ David  14 Sept 2011
 
 Charlie 15 Sep 2011
     - lib: fix compiler warning.
+    - vboxwrapper: fix compiler warning.
 
     lib/
         coproc.cpp
+    samples/
+        vboxwrapper/
+            vbox.cpp

samples/vboxwrapper/vbox.cpp

@@ -1078,7 +1078,7 @@ int VBOX_VM::get_vm_process_id(int& process_id) {
     pid = output.substr(pid_location, pid_length - pid_location);
     if (pid.size() <= 0) {
         fprintf(stderr, "%s no PID: location %d length %d\n",
-            boinc_msg_prefix(buf, sizeof(buf)), (int)pid_location, (int)pid_length - pid_location
+            boinc_msg_prefix(buf, sizeof(buf)), (int)pid_location, (int)(pid_length - pid_location)
         );
         return ERR_NOT_FOUND;
     }