Fix Bug# 610765 - test g_arg_info_may_be_null before accepting null
Fix Bug# 610765 - test g_arg_info_may_be_null before accepting null

Correct behaviour is to read gir hints on null for arguments - otherwise segfaults happen...
This does however break a few function calls, that have got broken gir notes,
eg. gtk_drag_source_set gtk_drag_dest_set and gst_bus_add_watch

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