JSONImporter: Free new schedule if found invalid

In case we detect that the schedule the user wants to import is invalid we
refuse it _and_ free the isl_maps containing it.

Another bug found thanks to Rafael.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@187339 91177308-0d34-0410-b5e6-96231b3b80d8