Fixed regression in 220ac5f causing em dashes to be considered invali…

…d filename chars

Fixes #2422. Since em dashes were considered invalid, and MP scenario names almost all have
em dashes, savegames were created without them, and thus the game couldn't match any when
generating entries for the Back to Turn... menu.

src/savegame.cpp

@@ -418,7 +418,7 @@ bool savegame::check_filename(const std::string& filename)
 
 bool savegame::is_illegal_file_char(char c)
 {
-	return c == '/' || c == '\\' || c == ':' || c < 0x20
+	return c == '/' || c == '\\' || c == ':' || (c >= 0x00 && c < 0x20)
 #ifdef _WIN32
 	|| c == '?' || c == '|' || c == '<' || c == '>' || c == '*' || c == '"'
 #endif