Fix harmless access to uninitialized memory.

When cache invalidations arrive while ri_LoadConstraintInfo() is busy
filling a new cache entry, InvalidateConstraintCacheCallBack() compares
the - not yet initialized - oidHashValue field with the to-be-invalidated
hash value. To fix, check whether the entry is already marked as invalid.

Andres Freund
docs:  9.4 release notes adjustments

Patch by Andres Freund, slight adjustments by me
Merge remote-tracking branch 'upstream/master'