[SPARK-10468] [ MLLIB ] Verify schema before Dataframe select API call

Loader.checkSchema was called to verify the schema after dataframe.select(...).
Schema verification should be done before dataframe.select(...)

Author: Vinod K C <vinod.kc@huawei.com>

Closes #8636 from vinodkc/fix_GaussianMixtureModel_load_verification.