GODRIVER-1918 Check for zero length in readstring (#613)

Author: benjirewis

x/bsonx/bsoncore/bsoncore.go

@@ -818,7 +818,7 @@ func readstring(src []byte) (string, []byte, bool) {
 	if !ok {
 		return "", src, false
 	}
-	if len(src[4:]) < int(l) {
+	if len(src[4:]) < int(l) || l == 0 {
 		return "", src, false
 	}
 

x/bsonx/bsoncore/value_test.go

@@ -109,6 +109,11 @@ func TestValue(t *testing.T) {
 			NewInsufficientBytesError([]byte{0x01, 0x02, 0x03, 0x04}, []byte{0x01, 0x02, 0x03, 0x04}),
 			nil,
 		},
+		{
+			"StringValue/Zero Length", Value.StringValue, Value{Type: bsontype.String, Data: []byte{0x00, 0x00, 0x00, 0x00}},
+			NewInsufficientBytesError([]byte{0x00, 0x00, 0x00, 0x00}, []byte{0x00, 0x00, 0x00, 0x00}),
+			nil,
+		},
 		{
 			"StringValue/Success", Value.StringValue, Value{Type: bsontype.String, Data: AppendString(nil, "hello, world!")},
 			nil,
@@ -124,6 +129,11 @@ func TestValue(t *testing.T) {
 			nil,
 			[]interface{}{string(""), false},
 		},
+		{
+			"StringValueOK/Zero Length", Value.StringValueOK, Value{Type: bsontype.String, Data: []byte{0x00, 0x00, 0x00, 0x00}},
+			nil,
+			[]interface{}{string(""), false},
+		},
 		{
 			"StringValueOK/Success", Value.StringValueOK, Value{Type: bsontype.String, Data: AppendString(nil, "hello, world!")},
 			nil,