Negative value can be returned from function is not being checked before being used improperly

Defect 86187 (STATIC_C)
Checker NEGATIVE_RETURNS (subcategory none)
File: /src/mongo/db/modules/enterprise/src/ldap/name_mapping/rewrite_rule.cpp
Function mongo::RewriteRule::_extractMatches(const pcrecpp::RE &, mongo::StringData) const
/src/mongo/db/modules/enterprise/src/ldap/name_mapping/rewrite_rule.cpp, line: 20
Assigning: unsigned variable "numCaptureGroups" = "NumberOfCapturingGroups".

        size_t numCaptureGroups = match.NumberOfCapturingGroups();