Fix zed.MapperLookupCache bug (#4877)

The recent switch in MapperLookupCache.Lookup to slices.Grow in #4864
introduces a bug because Reset truncates the cache without zeroing it,
allowing slices.Grow to expose old entries.  This wasn't a problem
previously because the method used to grow the cache zeroed old entries
before reusing their storage.

mapper.go

@@ -72,6 +72,7 @@ type MapperLookupCache struct {
 }
 
 func (m *MapperLookupCache) Reset(mapper *Mapper) {
+	clear(m.cache)
 	m.cache = m.cache[:0]
 	m.mapper = mapper
 }