It is currently possible as a non-superuser to create a new user or patch an existing user with arbitrary company over the API if FullMultipleCompanySupport is enabled.
Altough a highly unlikely scenario as the user needs permission to create API keys and new users, it is a bug that should get fixed.
Add a call to getIdForCurrentUser() to normalize the company_id if FullMultipleCompanySupport is enabled.