bpo-41056: Fix a possible MemoryError leak within zoneinfo. (GH-21007)

This was detected by our Coverity scan as a REVERSE_INULL issue. Automerge-Triggered-By: @gpshead

Author: gpshead

Misc/NEWS.d/next/Library/2020-06-20-18-33-03.bpo-41056.gTH4Bq.rst

@@ -0,0 +1 @@
+Fixed an instance where a MemoryError within the zoneinfo module might not be reported or not reported at its source. (found by Coverity)

Modules/_zoneinfo.c

@@ -278,13 +278,11 @@ zoneinfo_new(PyTypeObject *type, PyObject *args, PyObject *kw)
 
  instance =
  PyObject_CallMethod(weak_cache, "setdefault", "OO", key, tmp);
- ((PyZoneInfo_ZoneInfo *)instance)->source = SOURCE_CACHE;
-
  Py_DECREF(tmp);
-
  if (instance == NULL) {
  return NULL;
  }
+ ((PyZoneInfo_ZoneInfo *)instance)->source = SOURCE_CACHE;
  }
 
  update_strong_cache(type, key, instance);
@@ -1622,7 +1620,7 @@ parse_abbr(const char *const p, PyObject **abbr)
  }
 
  *abbr = PyUnicode_FromStringAndSize(str_start, str_end - str_start);
- if (abbr == NULL) {
+ if (*abbr == NULL) {
  return -1;
  }