gdb/python: Use reference not pointer in py-registers.c

gdb/python: Use reference not pointer in py-registers.c

Pedro's review comments arrived after I'd already committed this
change:

  commit f7306dac19c502232f766c3881313857915f330d
  Date:   Tue Jul 7 15:00:30 2020 +0100

      gdb/python: Reuse gdb.RegisterDescriptor objects where possible

See:

  https://sourceware.org/pipermail/gdb-patches/2020-July/170726.html

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* python/py-registers.c (gdbpy_register_object_data_init): Remove
	redundant local variable.
	(gdbpy_get_register_descriptor): Extract descriptor vector as a
	reference, not pointer, update code accordingly.