Revert "index: remove redundant storage of indexmap size"

This reverts commit f1c388c623.

For an uninitialized indexmap the returned size was `-1` which is
unexpected and could cause problems.

internal/index/indexmap.go

@@ -2,7 +2,6 @@ package index
 
 import (
 	"hash/maphash"
-	"math"
 
 	"github.com/restic/restic/internal/restic"
 )
@@ -18,7 +17,8 @@ import (
 // needs to be resized when the table grows, preventing memory usage spikes.
 type indexMap struct {
 	// The number of buckets is always a power of two and never zero.
-	buckets []uint
+	buckets    []uint
+	numentries uint
 
 	mh maphash.Hash
 
@@ -34,9 +34,9 @@ const (
 // using id as the key.
 func (m *indexMap) add(id restic.ID, packIdx int, offset, length uint32, uncompressedLength uint32) {
 	switch {
-	case m.len() == math.MaxUint: // Lazy initialization.
+	case m.numentries == 0: // Lazy initialization.
 		m.init()
-	case m.len() >= maxLoad*uint(len(m.buckets)):
+	case m.numentries >= maxLoad*uint(len(m.buckets)):
 		m.grow()
 	}
 
@@ -50,6 +50,7 @@ func (m *indexMap) add(id restic.ID, packIdx int, offset, length uint32, uncompr
 	e.uncompressedLength = uncompressedLength
 
 	m.buckets[h] = idx
+	m.numentries++
 }
 
 // foreach calls fn for all entries in the map, until fn returns false.
@@ -131,7 +132,7 @@ func (m *indexMap) init() {
 	m.newEntry()
 }
 
-func (m *indexMap) len() uint { return m.blockList.Size() - 1 }
+func (m *indexMap) len() uint { return m.numentries }
 
 func (m *indexMap) newEntry() (*indexEntry, uint) {
 	return m.blockList.Alloc()