Internal error for ILIKE_ESCAPE and special characters

[mrigger]

Consider the following testcase:

SELECT ILIKE_ESCAPE('a', '꧕', '�'); -- internal error: runtime error: index out of range [2] with length 1

Unexpectedly, the query results in an internal error with the following stacktrace:

ERROR: internal error: runtime error: index out of range [2] with length 1
SQLSTATE: XX000
DETAIL: stack trace:
github.com/cockroachdb/cockroach/pkg/util/errorutil/catch.go:29: ShouldCatch()
github.com/cockroachdb/cockroach/pkg/sql/opt/optbuilder/builder.go:169: func1()
runtime/panic.go:679: gopanic()
runtime/panic.go:75: goPanicIndex()
unicode/utf8/utf8.go:355: EncodeRune()
github.com/cockroachdb/cockroach/pkg/sql/sem/tree/eval.go:4862: replaceCustomEscape()
github.com/cockroachdb/cockroach/pkg/sql/sem/tree/eval.go:5090: Pattern()
github.com/cockroachdb/cockroach/pkg/sql/sem/tree/regexp_cache.go:62: GetRegexp()
github.com/cockroachdb/cockroach/pkg/sql/sem/tree/eval.go:2521: ConvertLikeToRegexp()
github.com/cockroachdb/cockroach/pkg/sql/sem/tree/eval.go:2503: MatchLikeEscape()
github.com/cockroachdb/cockroach/pkg/sql/sem/builtins/builtins.go:1194: func79()
github.com/cockroachdb/cockroach/pkg/sql/sem/builtins/builtins.go:4552: func1()
github.com/cockroachdb/cockroach/pkg/sql/sem/tree/eval.go:4008: Eval()
github.com/cockroachdb/cockroach/pkg/sql/opt/norm/fold_constants.go:353: FoldFunction()
github.com/cockroachdb/cockroach/pkg/sql/opt/norm/factory.og.go:13856: ConstructFunction()
github.com/cockroachdb/cockroach/pkg/sql/opt/optbuilder/scalar.go:476: buildFunction()
github.com/cockroachdb/cockroach/pkg/sql/opt/optbuilder/scalar.go:290: buildScalar()
github.com/cockroachdb/cockroach/pkg/sql/opt/optbuilder/project.go:171: buildProjectionList()
github.com/cockroachdb/cockroach/pkg/sql/opt/optbuilder/select.go:930: buildSelectClause()
github.com/cockroachdb/cockroach/pkg/sql/opt/optbuilder/select.go:850: buildSelectStmtWithoutParens()
github.com/cockroachdb/cockroach/pkg/sql/opt/optbuilder/select.go:823: func1()
github.com/cockroachdb/cockroach/pkg/sql/opt/optbuilder/with.go:29: processWiths()
github.com/cockroachdb/cockroach/pkg/sql/opt/optbuilder/select.go:822: buildSelect()
github.com/cockroachdb/cockroach/pkg/sql/opt/optbuilder/builder.go:253: buildStmt()
github.com/cockroachdb/cockroach/pkg/sql/opt/optbuilder/builder.go:217: buildStmtAtRoot()
github.com/cockroachdb/cockroach/pkg/sql/opt/optbuilder/builder.go:188: Build()
github.com/cockroachdb/cockroach/pkg/sql/plan_opt.go:420: buildExecMemo()
github.com/cockroachdb/cockroach/pkg/sql/plan_opt.go:162: makeOptimizerPlan()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_exec.go:795: makeExecPlan()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_exec.go:678: dispatchToExecutionEngine()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_exec.go:471: execStmtInOpenState()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_exec.go:101: execStmt()

I found this bug based on commit 031ab37f38a9095da3740c13a5ac03531f13608c.