Always add additional space if no more is available

Change-Id: If08b2d839489d40e977de794b13584fa66ff32c1

go/builder.go

@@ -198,7 +198,7 @@ func (b *Builder) Prep(size, additionalBytes int) {
 	alignSize &= (size - 1)
 
 	// Reallocate the buffer if needed:
-	for int(b.head) < alignSize+size+additionalBytes {
+	for int(b.head) <= alignSize+size+additionalBytes {
 		oldBufSize := len(b.Bytes)
 		b.growByteBuffer()
 		b.head += UOffsetT(len(b.Bytes) - oldBufSize)

tests/go_test.go

@@ -21,12 +21,13 @@ import (
 	"bytes"
 	"flag"
 	"fmt"
-	flatbuffers "github.com/google/flatbuffers/go"
 	"io/ioutil"
 	"os"
 	"reflect"
 	"sort"
 	"testing"
+
+	flatbuffers "github.com/google/flatbuffers/go"
 )
 
 var (
@@ -479,6 +480,20 @@ func CheckByteLayout(fail func(string, ...interface{})) {
 	b.EndVector(2)
 	check([]byte{2, 0, 0, 0, 2, 1, 0, 0}) // padding
 
+	// test 3b: 11xbyte vector matches builder size
+
+	b = flatbuffers.NewBuilder(12)
+	b.StartVector(flatbuffers.SizeByte, 8, 1)
+	start := []byte{}
+	check(start)
+	for i := 1; i < 12; i++ {
+		b.PrependByte(byte(i))
+		start = append([]byte{byte(i)}, start...)
+		check(start)
+	}
+	b.EndVector(8)
+	check(append([]byte{8, 0, 0, 0}, start...))
+
 	// test 4: 1xuint16 vector
 
 	b = flatbuffers.NewBuilder(0)