gdb/python: don't use PyObject_IsInstance in py-registers.c

gdb/python: don't use PyObject_IsInstance in py-registers.c

In python/py-registers.c we make use of PyObject_IsInstance.  The
PyObject_IsInstance can return -1 for an error, 0 for false, or 1 for
true.

In py-registers.c we treat the return value from PyObject_IsInstance
as a boolean, which means both -1 and 1 will be treated as true.

If PyObject_IsInstance returns -1 for an error, this will be treated
as true, we will then invoke undefined behaviour as the pyo_reg_id
object will be treated as a gdb.RegisterDescriptor, even though it
might not be.

I noticed that the gdb.RegisterDescriptor class does not have the
Py_TPFLAGS_BASETYPE flag, and therefore cannot be inherited from.  As
such, using PyObject_IsInstance is not necessary, we can use
PyObject_TypeCheck instead.  The PyObject_TypeCheck function only
returns 0 or 1, so we don't need to worry about the error case.

Approved-By: Tom Tromey <tom@tromey.com>