UnitTestFrameworkPkg/UnitTestLib: Check Suite pointer before use.

UnitTestFrameworkPkg/UnitTestLib: Check Suite pointer before use.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2530

The Suite pointer is used before check if it is valid,
correct it to check the validation before use.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Signed-off-by: GuoMinJ <newexplorerj@gmail.com>
Reviewed-by: Shenglei Zhang <shenglei.zhang@intel.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Bret Barkelew <bret.barkelew@microsoft.com>