Problem: Asserts in SHOW CREATE TABLE observed in RQG runs. Root cause
appears to be a failure to restore TABLE::read_set when returning
early due to errors in store_create_info().

Solution: Add a RAII scope guard which ensures the bitset is always
restored when leaving the scope.

Change-Id: I1033e05d6f2d3d8d6f729ce0c919f48a396820f0