debug/macho: use saferio to read symbol table strings

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

For golang#47653
Fixes golang#58603

Change-Id: I67fc45365c1a5b0b4b381f541bf2fee8ce8ddc3a
Reviewed-on: https://go-review.googlesource.com/c/go/+/469895
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

src/debug/macho/file.go

@@ -323,8 +323,8 @@ func NewFile(r io.ReaderAt) (*File, error) {
 			if err := binary.Read(b, bo, &hdr); err != nil {
 				return nil, err
 			}
-			strtab := make([]byte, hdr.Strsize)
-			if _, err := r.ReadAt(strtab, int64(hdr.Stroff)); err != nil {
+			strtab, err := saferio.ReadDataAt(r, uint64(hdr.Strsize), int64(hdr.Stroff))
+			if err != nil {
 				return nil, err
 			}
 			var symsz int