fix race condition in Reader due to concurrent map write

```
fatal error: concurrent map writes
github.com/richardlehane/msoleps.addDefaults(...)
	/go/pkg/mod/github.com/richardlehane/msoleps@v1.0.3/sets.go:20
github.com/richardlehane/msoleps.(*Reader).start(0xc001bb1f78, {0xc8a180?, 0xc00037be80?})
	/go/pkg/mod/github.com/richardlehane/msoleps@v1.0.3/msoleps.go:125 +0x2de
github.com/richardlehane/msoleps.(*Reader).Reset(...)
	/go/pkg/mod/github.com/richardlehane/msoleps@v1.0.3/msoleps.go:76
...
```

.travis.yml

@@ -2,4 +2,7 @@ language: go
 go:
   - 1.2
   - stable
-  - tip
+  - tip
+
+script:
+  - go test -v -race ./...

msoleps.go

@@ -115,17 +115,18 @@ func (r *Reader) start(rdr io.Reader) error {
 		plen += len(psb.idsOffs)
 	}
 	r.Property = make([]*Property, plen)
-	dict, ok := propertySets[pss.fmtidA]
-	if !ok {
-		dict = ps.dict
-		if dict == nil {
-			dict = make(map[uint32]string)
+	dicta := make(map[uint32]string)
+	if propertySet, ok := propertySets[pss.fmtidA]; ok {
+		for k, v := range propertySet {
+			dicta[k] = v
 		}
+	} else if ps.dict != nil {
+		dicta = ps.dict
 	}
-	dict = addDefaults(dict)
+	dicta = addDefaults(dicta)
 	for i, v := range ps.idsOffs {
 		r.Property[i] = &Property{}
-		r.Property[i].Name = dict[v.id]
+		r.Property[i].Name = dicta[v.id]
 		// don't try to evaluate dictionary property
 		if v.id == 0x00000000 {
 			r.Property[i].T = types.Null{}
@@ -142,18 +143,19 @@ func (r *Reader) start(rdr io.Reader) error {
 	if pss.numPropertySets != 2 {
 		return nil
 	}
-	dict, ok = propertySets[pss.fmtidB]
-	if !ok {
-		dict = psb.dict
-		if dict == nil {
-			dict = make(map[uint32]string)
+	dictb := make(map[uint32]string)
+	if propertySet, ok := propertySets[pss.fmtidB]; ok {
+		for k, v := range propertySet {
+			dictb[k] = v
 		}
+	} else if psb.dict != nil {
+		dictb = psb.dict
 	}
-	dict = addDefaults(dict)
+	dictb = addDefaults(dictb)
 	for i, v := range psb.idsOffs {
 		i += len(ps.idsOffs)
 		r.Property[i] = &Property{}
-		r.Property[i].Name = dict[v.id]
+		r.Property[i].Name = dictb[v.id]
 		// don't try to evaluate dictionary property
 		if v.id == 0x00000000 {
 			r.Property[i].T = types.Null{}

msoleps_test.go

@@ -2,6 +2,7 @@ package msoleps
 
 import (
 	"os"
+	"sync"
 	"testing"
 )
 
@@ -21,6 +22,20 @@ func testFile(t *testing.T, path string) *Reader {
 	return doc
 }
 
+func TestReader_concurrency(t *testing.T) {
+	var wg sync.WaitGroup
+	wg.Add(2)
+	go func() {
+		defer wg.Done()
+		_ = testFile(t, testDocSum)
+	}()
+	go func() {
+		defer wg.Done()
+		_ = testFile(t, testDocSum)
+	}()
+	wg.Wait()
+}
+
 func TestDocSum(t *testing.T) {
 	doc := testFile(t, testDocSum)
 	if len(doc.Property) != 12 {