PYTHON-3443 Remove redundant code to avoid Coverity warnings (#1228)

Author: ShaneHarvey

bson/_cbsonmodule.c

@@ -304,19 +304,15 @@ static int millis_from_datetime_ms(PyObject* dt, long long* out){
     long long millis;
 
     if (!(ll_millis = PyNumber_Long(dt))){
-        if (PyErr_Occurred()) { // TypeError
-            return 0;
-        }
-    }
-
-    if ((millis = PyLong_AsLongLong(ll_millis)) == -1){
-        if (PyErr_Occurred()) { /* Overflow */
-            PyErr_SetString(PyExc_OverflowError,
-                            "MongoDB datetimes can only handle up to 8-byte ints");
-            return 0;
-        }
+        return 0;
     }
+    millis = PyLong_AsLongLong(ll_millis);
     Py_DECREF(ll_millis);
+    if (millis == -1 && PyErr_Occurred()) { /* Overflow */
+        PyErr_SetString(PyExc_OverflowError,
+                        "MongoDB datetimes can only handle up to 8-byte ints");
+        return 0;
+    }
     *out = millis;
     return 1;
 }
@@ -2081,7 +2077,7 @@ static PyObject* get_value(PyObject* self, PyObject* name, const char* buffer,
                         millis = max_millis;
                     }
                     // Continues from here to return a datetime.
-                } else if (dt_auto) {
+                } else { // dt_auto
                     if (millis < min_millis || millis > max_millis){
                         value = datetime_ms_from_millis(self, millis);
                         break; // Out-of-range so done.