Don't crash when Amending hosts that don't yet have their provider attribute set

This can happen when merging hosts in providers. Wasn't triggered
before, because those merges should be of completely disjoint sets, but
we have some garbage in consul kv now that triggered this.

Author: seveas

host.go

@@ -181,7 +181,12 @@ func (h *Host) Amend(h2 *Host) {
 	if h.Address == "" {
 		h.Address = h2.Address
 	}
-	h.Attributes["herd_provider"] = append(h.Attributes["herd_provider"].([]string), h2.Attributes["herd_provider"].([]string)[0])
+	if h2.Attributes["herd_provider"] != nil {
+		if h.Attributes["herd_provider"] == nil {
+			h.Attributes["herd_provider"] = make([]string, 0)
+		}
+		h.Attributes["herd_provider"] = append(h.Attributes["herd_provider"].([]string), h2.Attributes["herd_provider"].([]string)[0])
+	}
 	for attr, value := range h2.Attributes {
 		if attr == "herd_provider" {
 			continue

host_test.go

@@ -36,3 +36,9 @@ func TestHostDeserialization(t *testing.T) {
 		t.Errorf("float attribute did not survive the json trip: %v", host.Attributes)
 	}
 }
+
+func TestAmendWithoutProvider(t *testing.T) {
+	h := NewHost("test-host.herd.ci", "127.0.0.1", HostAttributes{})
+	h2 := NewHost("test-host.herd.ci", "127.0.0.1", HostAttributes{})
+	h.Amend(h2)
+}

version.go

@@ -7,7 +7,7 @@ import (
 const (
 	majorVersion = 0
 	minorVersion = 12
-	patchVersion = 0
+	patchVersion = 1
 )
 
 func Version() string {