Update reference counting

from Barry Warsaw:

- It's possible the code jumps to error: after pkgname is decref'd. I
  think that decref should be deferred until just before the good-path
  return.

- pkgname_or_unknown will be a new reference if pkgname is NULL, or it
  will steal the pkgname reference if it's not. In either case, the error
  stanza should decref pkgname_or_unknown, which will take care of
  ensuring the object gets decref for any goto error path.

- pkgpath doesn't get decref'd. I think you should Py_XDECREF it before
  the return NULL just in case pkgpath is itself NULL.

Author: Carreau

Python/ceval.c

@@ -5009,7 +5009,6 @@ import_from(PyObject *v, PyObject *name)
         goto error;
     }
     fullmodname = PyUnicode_FromFormat("%U.%U", pkgname, name);
-    Py_DECREF(pkgname);
     if (fullmodname == NULL) {
         return NULL;
     }
@@ -5018,6 +5017,7 @@ import_from(PyObject *v, PyObject *name)
     if (x == NULL) {
         goto error;
     }
+    Py_DECREF(pkgname);
     Py_INCREF(x);
     return x;
  error:
@@ -5041,6 +5041,8 @@ import_from(PyObject *v, PyObject *name)
             pkgname, pkgpath);
     }
 
+    Py_XDECREF(pkgname_or_unknown);
+    Py_XDECREF(pkgpath);
     return NULL;
 }