runtime/pprof: preallocate slice in pprof.go

To prevent slice growth when appending.

Change-Id: I2cdb9b09bc33f63188b19573c8b9a77601e63801
Reviewed-on: https://go-review.googlesource.com/23783
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/pprof/pprof.go

@@ -202,7 +202,7 @@ func Profiles() []*Profile {
 	lockProfiles()
 	defer unlockProfiles()
 
-	var all []*Profile
+	all := make([]*Profile, 0, len(profiles.m))
 	for _, p := range profiles.m {
 		all = append(all, p)
 	}
@@ -293,7 +293,7 @@ func (p *Profile) WriteTo(w io.Writer, debug int) error {
 	}
 
 	// Obtain consistent snapshot under lock; then process without lock.
-	var all [][]uintptr
+	all := make([][]uintptr, 0, len(p.m))
 	p.mu.Lock()
 	for _, stk := range p.m {
 		all = append(all, stk)