Fix zed.MapperLookupCache.Lookup OOM on bad ID (#4864)

Passing a large type ID to Lookup can cause it to allocate an arbirary
amount of memory.  Fix by not growing the cache if the ID is unknown to
the underlying zed.Mapper.

Closes #4858.

mapper.go

@@ -77,12 +77,19 @@ func (m *MapperLookupCache) Reset(mapper *Mapper) {
 }
 
 func (m *MapperLookupCache) Lookup(id int) Type {
-	if id >= len(m.cache) {
-		m.cache = append(m.cache, make([]Type, id+1-len(m.cache))...)
-	} else if typ := m.cache[id]; typ != nil {
-		return typ
+	if id < len(m.cache) {
+		if typ := m.cache[id]; typ != nil {
+			return typ
+		}
 	}
 	typ := m.mapper.Lookup(id)
+	if typ == nil {
+		// To prevent OOM, don't grow cache if id is unknown.
+		return nil
+	}
+	if id >= len(m.cache) {
+		m.cache = slices.Grow(m.cache[:0], id+1)[:id+1]
+	}
 	m.cache[id] = typ
 	return typ
 }