debug/elf: check for multiplication overflow for shnum * shentsize

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

For #47653
Fixes #70584

Change-Id: I8a69a27dcb5b258b88f8e01ebaf0ec20cfcd489b
Reviewed-on: https://go-review.googlesource.com/c/go/+/632035
Reviewed-by: Veronica Silina <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Tobias Klauser <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: ianlancetaylor

src/debug/elf/file.go

@@ -497,6 +497,9 @@ func NewFile(r io.ReaderAt) (*File, error) {
 	if c < 0 {
 		return nil, &FormatError{0, "too many sections", shnum}
 	}
+	if shnum > 0 && ((1<<64)-1)/uint64(shnum) < uint64(shentsize) {
+		return nil, &FormatError{0, "section header overflow", shnum}
+	}
 	f.Sections = make([]*Section, 0, c)
 	names := make([]uint32, 0, c)
 	shdata, err := saferio.ReadDataAt(sr, uint64(shnum)*uint64(shentsize), shoff)