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_test.go

@@ -37,4 +37,9 @@ func TestIntegration(t *testing.T) {
 		opt.QuickTestOK = true
 	}
 	fstests.Run(t, &opt)
+	// test again with MaxAge = 0
+	if *fstest.RemoteName == "" {
+		opt.ExtraConfig = append(opt.ExtraConfig, fstests.ExtraConfigItem{Name: "TestHasher", Key: "max_age", Value: "0"})
+		fstests.Run(t, &opt)
+	}
 }