Validate correct field name in RoleName::parseFromBSON()

Title: Denial of Service when processing malformed Role names

Description: Incorrect validation of user input in the role name parser may lead to use of uninitialized memory allowing an unauthenticated attacker to use a specially crafted request to cause a denial of service. This issue affects: MongoDB Inc. MongoDB Server v4.4 versions prior to 4.4.0-rc12; v4.2 versions prior to 4.2.9.

CVSS score: 7.5 using the following scoring metrics:
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CWE: CWE-475: Undefined Behavior for Input to API

Affected versions:

This issue affects - MongoDB Inc. MongoDB Server:

• v4.4 versions prior to 4.4.0-rc12;
• v4.2 versions prior to 4.2.9.