[dev.link] cmd/objdump: switch to using NewReaderFromBytes

Convert the object file dumper to use NewReaderFromBytes when
reading new object files, as opposed to NewReader.

Change-Id: I9f5e0356bd21c16f545cdd70262e983a2ed38bfc
Reviewed-on: https://go-review.googlesource.com/c/go/+/201441
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/internal/goobj/read.go

@@ -502,12 +502,15 @@ func (r *objReader) parseObject(prefix []byte) error {
 	}
 	// TODO: extract OS + build ID if/when we need it
 
-	r.readFull(r.tmp[:8])
-	if bytes.Equal(r.tmp[:8], []byte("\x00go114LD")) {
-		r.offset -= 8
+	p, err := r.peek(8)
+	if err != nil {
+		return err
+	}
+	if bytes.Equal(p, []byte("\x00go114LD")) {
 		r.readNew()
 		return nil
 	}
+	r.readFull(r.tmp[:8])
 	if !bytes.Equal(r.tmp[:8], []byte("\x00go114ld")) {
 		return r.error(errCorruptObject)
 	}

src/cmd/internal/goobj/readnew.go

@@ -15,7 +15,11 @@ import (
 // the data to the current goobj API.
 func (r *objReader) readNew() {
 	start := uint32(r.offset)
-	rr := goobj2.NewReader(r.f, start)
+
+	length := r.limit - r.offset
+	objbytes := make([]byte, length)
+	r.readFull(objbytes)
+	rr := goobj2.NewReaderFromBytes(objbytes, false)
 	if rr == nil {
 		panic("cannot read object file")
 	}