ccmake: Delete main form on exit after a failed configure

When Configure() or Generate() failed with -2 (e.g. no CMakeLists.txt
in the source directory) they returned while CurrentForm still pointed
at the message form.  main() then deleted that form instead of the main
form, leaking the main form together with its cmake instance.

Source/CursesDialog/ccmake.cxx

@@ -186,8 +186,8 @@ int main(int argc, char const* const* argv)
   clear();
   touchwin(stdscr);
   endwin();
-  delete cmCursesForm::CurrentForm;
   cmCursesForm::CurrentForm = nullptr;
+  delete myform;
 
   std::cout << std::endl << std::endl;
 

Source/CursesDialog/cmCursesMainForm.cxx

@@ -537,6 +537,7 @@ int cmCursesMainForm::Configure(int noconfigure)
     // If they typed the wrong source directory, we report
     // an error and exit
     if (retVal == -2) {
+      CurrentForm = this;
       return retVal;
     }
   }
@@ -590,6 +591,7 @@ int cmCursesMainForm::Generate()
     // If they typed the wrong source directory, we report
     // an error and exit
     if (retVal == -2) {
+      CurrentForm = this;
       return retVal;
     }
   }