Avoid questionable casts in py-symtab.c

Avoid questionable casts in py-symtab.c

py-symtab.c has some questionable casts of Py_None to symtab_object*.
This patch avoids these casts by instead using downcasts at the
appropriate places.

gdb/ChangeLog
2019-01-03  Tom Tromey  <tom@tromey.com>

	* python/py-symtab.c (salpy_str): Update.
	(struct salpy_sal_object) <symtab>: Now a PyObject.
	(salpy_dealloc): Update.
	(del_objfile_sal): Use gdbpy_ref.