cmd/link: fix size of filetab slice

The linker writes the number of file symbols (Nhistfile) to the filetab slice
and then Nhistfile offsets -- which means the slice contains Nhistfile+1
entries, not just Nhistfile.

I think this bug has been around since at least 1.4 but it's easier to trigger
with shared libraries and a tiny binary that only has a couple of functions in
it -- try go install -buildmode=shared std && go run -linkshared test/fixedbugs/issue4388.go.

Change-Id: I6c0f01f1e607b9b2b96872e37ffce81281911504
Reviewed-on: https://go-review.googlesource.com/16342
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/link/internal/ld/symtab.go

@@ -498,8 +498,8 @@ func symtab() {
 	adduint(Ctxt, moduledata, uint64(pclntabNfunc+1))
 	// The filetab slice
 	Addaddrplus(Ctxt, moduledata, Linklookup(Ctxt, "runtime.pclntab", 0), int64(pclntabFiletabOffset))
-	adduint(Ctxt, moduledata, uint64(Ctxt.Nhistfile))
-	adduint(Ctxt, moduledata, uint64(Ctxt.Nhistfile))
+	adduint(Ctxt, moduledata, uint64(Ctxt.Nhistfile)+1)
+	adduint(Ctxt, moduledata, uint64(Ctxt.Nhistfile)+1)
 	// findfunctab
 	Addaddr(Ctxt, moduledata, Linklookup(Ctxt, "runtime.findfunctab", 0))
 	// minpc, maxpc