Add missing incref when creating Inferior Python object

The test py-inferior.exp fails when using a debug build of Python 3.6.  I don't
see it failing with my system's default Python, but it might be related to the
different memory allocation scheme used when doing a build with pydebug.

The issue is that we are missing a Py_INCREF in
inferior_to_inferior_object.  The PyObject_New function initializes the
object with a refcount of 1.  If we assume that this refcount
corresponds to the reference we are returning, then we are missing an
incref for the reference in the inferior data.

The counterpart for the incref that corresponds to the reference in the
inferior data is in py_free_inferior, in the form the gdbpy_ref instance.

Here's how I can get it to crash (with some debug output):

  $ ./gdb -nx -ex "set debug python 1"
  (gdb) add-inferior
  Added inferior 2
  (gdb) python infs = gdb.inferiors()
  Creating Python Inferior object inf = 1
  Creating Python Inferior object inf = 2
  (gdb) remove-inferiors 2
  py_free_inferior inf = 2
  infpy_dealloc inf = <unknown>
  (gdb) python infs = None
  Fatal Python error: Objects/tupleobject.c:243 object at 0x7f9cf1a568d8 has negative ref count -1

  Current thread 0x00007f9cf1b68780 (most recent call first):
    File "<string>", line 1 in <module>
  [1]    408 abort (core dumped)  ./gdb -nx -ex "set debug python 1"

After having created the inferiors object, their refcount is 1 (which
comes from PyObject_New), but it should be two.  The gdb inferior object
has a reference and the "infs" list has a reference.

When invoking remove-inferiors, py_free_inferior gets called.  It does
the decref that corresponds to the reference that the gdb inferior
object kept.  At this moment, the refcount drops to 0 and the object
gets deallocated, even though the "infs" list still has a reference.
When we set "infs" to None, Python tries to decref the already zero
refcount and the assert triggers.

With this patch, it looks better:

  (gdb) add-inferior
  Added inferior 2
  (gdb) python infs = gdb.inferiors()
  Creating Python Inferior object inf = 1
  Creating Python Inferior object inf = 2
  (gdb) remove-inferiors 2
  py_free_inferior inf = 2
  (gdb) python infs = None
  infpy_dealloc inf = <unknown>

gdb/ChangeLog:

	* python/py-inferior.c (inferior_to_inferior_object): Increment reference
	count when creating the object.

1 files changed, 5 insertions, 2 deletions

diff --git a/gdb/python/py-inferior.c b/gdb/python/py-inferior.c
index 3d2cb1d..f6a24a0 100644
--- a/gdb/python/py-inferior.c
+++ b/gdb/python/py-inferior.c
@@ -223,11 +223,14 @@ inferior_to_inferior_object (struct inferior *inferior)
       inf_obj->threads = NULL;
       inf_obj->nthreads = 0;
 
+      /* PyObject_New initializes the new object with a refcount of 1.  This
+	 counts for the reference we are keeping in the inferior data.  */
       set_inferior_data (inferior, infpy_inf_data_key, inf_obj);
 
     }
-  else
-    Py_INCREF ((PyObject *)inf_obj);
+
+  /* We are returning a new reference.  */
+  Py_INCREF ((PyObject *)inf_obj);
 
   return (PyObject *) inf_obj;
 }