vulncheck/internal/gosym: fix bug on 32-bit systems

I was multiplying an offset by 8, instead of the size of a pointer.

Fixes golang/go#52218.

Change-Id: I396c833e52a96aff34b89e4e53053d15b4a0d1cd
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/398757
Trust: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

vulncheck/internal/gosym/pclntab.go

@@ -579,7 +579,7 @@ func (f funcData) funcdataOffset(i uint8) uint32 {
 	} else {
 		off = f.fieldOffset(10) + // skip fixed part of _func
 			f.npcdata()*4
-		off += uint32(i) * 8
+		off += uint32(i) * f.t.ptrsize
 	}
 	return f.t.binary.Uint32(f.data[off:])
 }