Coverity detected a potential out-of-bounds read and it does look like a real problem.

Incorrect values read from a different memory region will cause incorrect computations. Out-of-bounds read from a buffer
/src/third_party/wiredtiger/src/session/session_dhandle.c:404: OVERRUN 135341 Checking "cval.len == 0UL" implies that "cval.len" is at least 1 on the false branch.
/src/third_party/wiredtiger/src/session/session_dhandle.c:496: OVERRUN 135341 Checking "cval.len > strlen("WiredTigerCheckpoint")" implies that "cval.len" is at least 21 on the true branch.
/src/third_party/wiredtiger/src/session/session_dhandle.c:505: OVERRUN 135341 Overrunning array ""WiredTigerCheckpoint"" of 21 bytes at byte offset 21 using index "cval.len" (which evaluates to 21).