Fixed bug found by KroArtem where crashes occur if you specify an invalid gp name on the terminal

git-svn-id: svn+ssh://svn.code.sf.net/p/supertuxkart/code/main/trunk@9541 178a84e3-b1eb-0310-8ba1-8eac791a3b58

src/main.cpp

@@ -626,6 +626,13 @@ int handleCmdLine(int argc, char **argv)
         {
             race_manager->setMajorMode(RaceManager::MAJOR_MODE_GRAND_PRIX);
             const GrandPrixData *gp = grand_prix_manager->getGrandPrix(argv[i+1]);
+            
+            if (gp == NULL)
+            {
+                fprintf(stderr, "There is no GP named '%s'\n", argv[i+1]);
+                return 0;
+            }
+            
             race_manager->setGrandPrix(*gp);
             i++;
         }