Fix refcount leak in PyUnicode_EncodeCharmap(). The bug surfaces

Fix refcount leak in PyUnicode_EncodeCharmap(). The bug surfaces
when an encoding error occurs and the callback name is unknown,
i.e. when the callback has to be called. The problem was that
the fact that the callback has already been looked up was only
recorded in a local variable in charmap_encoding_error(), because
charmap_encoding_error() got it's own copy of the errorHandler
pointer instead of a pointer to the pointer in
PyUnicode_EncodeCharmap().