Fix crash when there are 0 font families.

Running Tux Paint under macOS in Korean mode (and some other languages) causes
it to crash, unless "load system fonts" option is also enabled. Some
investigation showed the issue occurs when num_font_families is 0 in fonts.c.
This change removes the assumption from the code that assumes num_font_families
has at least one record.

macos/README.txt

@@ -7,8 +7,8 @@ Starting with 0.9.23, however, Tux Paint for macOS is built as though it were a
 Linux application.
 
 
-REQUIREMENTS
-------------
+PREREQUISITES
+-------------
 Although Tux Paint is built without the XCode IDE, XCode itself is still required
 to build Tux Paint.  Download it from the App Store, and launch it once to
 accept its license agreements.  Also install XCode command line tools using the

src/fonts.c

@@ -900,7 +900,7 @@ static void groupfonts(void)
   qsort(user_font_families, num_font_families, sizeof user_font_families[0], compar_fontscore);
   //printf("groupfonts() qsort(user_font_families 2...)\n");
   //fflush(stdout);
-  if (user_font_families[0]->score < 0)
+  if (num_font_families > 0 && user_font_families[0]->score < 0)
     printf("sorted the wrong way, or all fonts were unusable\n");
 #if 0
 // THREADED_FONTS
@@ -1007,8 +1007,18 @@ static void loadfonts(SDL_Surface * screen, const char *const dir)
   free(homedirdir);
 #endif
 
+#ifdef DEBUG
+  printf("Grouping fonts...\n");
+  fflush(stdout);
+#endif
+
   groupfonts();
 
+#ifdef DEBUG
+  printf("Finished loading the fonts\n");
+  fflush(stdout);
+#endif
+
   font_thread_done = 1;
   waiting_for_fonts = 0;
   // FIXME: need a memory barrier here