Segfault/Assertion error in DBImplReadOnly::GetImpl when calling GetMergeOperands

[chengji77]

Expected behavior

GetMergeOperands should work when DB is opened in read only mode.

Actual behavior

Segfault in DBImplReadOnly::GetImpl when build in release mode.

When compiled using debug mode, this assert failed:

rocksdb/db/db_impl/db_impl_readonly.cc

Line 34 in 18cecb9

assert(get_impl_options.value != nullptr ||

Steps to reproduce the behavior

RocksDB v9.7.4
Open a db in read only mode and call db->GetMergeOperands

It might be a regression when adding wide column support. We had an older build using v8.6.7 and there was no crash.