debug/macho: use saferio to allocate load command slice

Avoid allocating large amounts of memory for corrupt input.

No test case because the problem can only happen for invalid data.
Let the fuzzer find cases like this.

Fixes #54780

Change-Id: Icdacb16bef7d29ef431da52e6d1da4e883a3e050
Reviewed-on: https://go-review.googlesource.com/c/go/+/427434
Run-TryBot: Tobias Klauser <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Tobias Klauser <[email protected]>

Author: tklauser

src/debug/macho/file.go

@@ -249,8 +249,8 @@ func NewFile(r io.ReaderAt) (*File, error) {
 	if f.Magic == Magic64 {
 		offset = fileHeaderSize64
 	}
-	dat := make([]byte, f.Cmdsz)
-	if _, err := r.ReadAt(dat, offset); err != nil {
+	dat, err := saferio.ReadDataAt(r, uint64(f.Cmdsz), offset)
+	if err != nil {
 		return nil, err
 	}
 	c := saferio.SliceCap([]Load{}, uint64(f.Ncmd))