UPBGE: Check for invalid id.lib in GetFontId.

Object not loaded from a library have a id.lib to NULL, they use
KG_GetMainPath() so.

Fix issue #418.

Author: panzergame

source/gameengine/Ketsji/KX_FontObject.cpp

@@ -261,7 +261,7 @@ int GetFontId(VFont *vfont)
 	// convert from absolute to relative
 	char expanded[FILE_MAX];
 	BLI_strncpy(expanded, filepath, FILE_MAX);
-	BLI_path_abs(expanded, vfont->id.lib->filepath);
+	BLI_path_abs(expanded, vfont->id.lib ? vfont->id.lib->name : KX_GetMainPath().c_str());
 
 	fontid = BLF_load(expanded);