replay: die when --onto does not peel to a commit

replay: die when --onto does not peel to a commit

The `peel_committish()` function calls `repo_peel_to_type()` to convert
the given object to a commit, but does not check the return value. When
the object exists but cannot be peeled to a commit (e.g., a tree or blob
OID is passed as --onto), the return value is NULL. Add an explicit NULL
check and die with a descriptive message in that case.

Pointed out by Coverity.

Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>