Bug#792785: segfault for Back button

[Alexey Loginov]

There is patch from Mageia Linux in attachment.

diff -ur tuxmath_w_fonts-2.0.3.orig/src/menu.c tuxmath_w_fonts-2.0.3/src/menu.c
--- tuxmath_w_fonts-2.0.3.orig/src/menu.c	2011-05-04 04:15:06.000000000 +0400
+++ tuxmath_w_fonts-2.0.3/src/menu.c	2015-10-15 21:41:49.000000000 +0300
@@ -252,6 +252,8 @@
     if (Opts_GetGlobalOpt(MENU_SOUND))
       playsound(SND_POP);
 
+    if (chosen_lesson>=num_lessons) return RUN_MAIN_MENU;
+
     /* Re-read global settings first in case any settings were */
     /* clobbered by other lesson or arcade games this session: */
     read_global_config_file();