stopped allowed disk usage from returning negative numbers

svn path=/trunk/boinc/; revision=968

client/client_state.C

@@ -334,6 +334,7 @@ int CLIENT_STATE::allowed_disk_usage(double& size) {
     // Return the minimum of the three
     //
     size = min(min(global_prefs.disk_max_used_gb*1e9, percent_space), min_val);
+	if(size < 0) size = 0;
     return 0;
 }