[dev.link] cmd/link: add in change missed from code review

Incorporate a change suggested by Cherry for CL 201721 that I missed
accidentally.

Change-Id: I65e6532e78888505573169e56bc4ace9a0f8c510
Reviewed-on: https://go-review.googlesource.com/c/go/+/202760
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/link/internal/loader/loader.go

@@ -459,7 +459,7 @@ func (relocs *Relocs) At(j int) Reloc {
 // larger slice will be allocated. Final slice is returned.
 func (relocs *Relocs) ReadAll(dst []Reloc) []Reloc {
 	if relocs.Count == 0 {
-		return dst
+		return dst[:0]
 	}
 
 	if cap(dst) < relocs.Count {