[base] Insert explict cast for GCC 4.6 in PIC mode. * src/base/ftinit.c (FT_Add_Default_Modules): Under PIC configuration, FT_DEFAULT_MODULES_GET returns FT_Module_Class** pointer, GCC 4.6 warns that const FT_Module_Class* const* variable is warned as inappropriate to store it. To calm it, explicit cast is inserted. Also `library' is checked to prevent the NULL pointer dereference in FT_DEFAULT_MODULES_GET.