make SDL_strcasecmp standard compliant

src/stdlib/SDL_string.c

@@ -1118,8 +1118,8 @@ SDL_strcasecmp(const char *str1, const char *str2)
         ++str1;
         ++str2;
     }
-    a = SDL_toupper(*str1);
-    b = SDL_toupper(*str2);
+    a = SDL_toupper((unsigned char) *str1);
+    b = SDL_toupper((unsigned char) *str2);
     return (int) ((unsigned char) a - (unsigned char) b);
 #endif /* HAVE_STRCASECMP */
 }