hasher: fix invalid memory address error when MaxAge == 0

When f.opt.MaxAge == 0, f.db is never set, however several methods later assume
it is set and attempt to access it, causing an invalid memory address error.
This change fixes the issue in a few spots (there may still be others I haven't
yet encountered.)

backend/hasher/hasher_internal_test.go

@@ -60,9 +60,11 @@ func (f *Fs) testUploadFromCrypt(t *testing.T) {
 	assert.NotNil(t, dst)
 
 	// check that hash was created
-	hash, err = f.getRawHash(ctx, hashType, fileName, anyFingerprint, longTime)
-	assert.NoError(t, err)
-	assert.NotEmpty(t, hash)
+	if f.opt.MaxAge > 0 {
+		hash, err = f.getRawHash(ctx, hashType, fileName, anyFingerprint, longTime)
+		assert.NoError(t, err)
+		assert.NotEmpty(t, hash)
+	}
 	//t.Logf("hash is %q", hash)
 	_ = operations.Purge(ctx, f, dirName)
 }